United Nations (CNN) -- Syrian President Bashar al-Assad has rebuffed the efforts of the joint U.N.-Arab League special envoy Kofi Annan to mediate an end to the violence, three U.S. administration officials told CNN on Tuesday.

The sources said al-Assad replied to Annan's proposals by saying he doesn't recognize Annan as the Arab League's representative and he will not do anything until the opposition lays down its arms.

Annan, a former U.N. secretary-general, had said earlier in a statement that he left "concrete proposals" with Syrian government officials after meeting over the weekend with al-Assad in Damascus. On Tuesday, before receiving the response, Annan met in Turkey with government officials and Syrian opposition members.

"Once I receive their answer, we will know how to react," Annan had said. "But let me say that the killing and the violence must stop. The Syrian people have gone through (a lot). They deserve better."

Annan met Tuesday with Burhan Ghalioun, chairman of the Syrian National Council, which has been representing the opposition in international meetings. The discussions were "productive," and the council promised "full cooperation" in upcoming efforts, Annan said.

But productivity has been difficult to measure. Repeated diplomatic efforts to resolve the Syrian conflict have failed. The U.N. Security Council took up the issue Monday, with the United States and Britain pushing for quick action on a resolution and Russia warning against blaming just one side.

U.S. Secretary of State Hillary Clinton, who took part in that meeting, said Tuesday, "We continue to apply pressure on Assad and his regime in Syria to stop the brutality and we work with the opposition and like-minded countries to try to help them be in a position to be part of a successful political transition."

On Monday, Clinton called on "all nations, even those who have previously blocked our efforts" to speak with one voice in calling for the killings of civilians to end and a transition to democracy to begin.

Russia and China vetoed a Security Council resolution last month that a number of other countries had argued could have helped stop the violence. The resolution would have condemned al-Assad and called on him to step aside.

Both Russia and China have major trade deals with Syria.

Valerie Amos, U.N. humanitarian chief who visited Syria last week, described the situation there as "clear deadlock."

Amos said she recently returned from a refugee camp across Syria's border with Turkey, where she spoke with displaced Syrians "who were very angry about what's happening in Syria and being abandoned by the international community."

Opposition activists declared Tuesday a day of mourning across Syria as the death toll from nearly a year of government attacks grew.

More than 8,000 people have been killed in the conflict, including many women and children, said Nassir Abdulaziz Al-Nasser, president of the U.N. General Assembly. Opposition activists have put the toll at more than 9,000.

"Violations of human rights are widespread and systematic," Al-Nasser said Monday. "The international community has a responsibility to act."

The call for a day of mourning followed what activists described as a massacre over the weekend in Homs, where dozens of women and children were reportedly stabbed and burned to death.

Al-Assad's regime insists "armed terrorist groups," which it routinely blames for the violence, were behind the killings in the Karm al-Zaytoun neighborhood. The state-run Syrian Arab News Agency said Tuesday that such groups had "committed a most atrocious massacre against women, children and elderly citizens in Karm al-Zaytoun neighborhood and mutilated their bodies in order to put pressure to elicit international stances against Syria."

Arab League Secretary-General Nabil Elaraby called Tuesday for "an international neutral investigation into the massacres of Homs, Hama, Idlib, Baba Amr, and other areas where dozens of citizens including women and children were killed, leaving many questions on who committed them as they are clearly crime(s) against humanity," an Arab League official said.

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"He called on the Syrian government to cooperate with the U.N. Human Rights Commission and other parties involved in this international probe in order to pinpoint who is responsible for these brutal crimes."

At least 45 people were killed Tuesday across Syria, according to the opposition group Local Coordination Committees of Syria, or LCC. The deaths included 12 in Homs, nine in Damascus suburbs and at least 12 in Idlib, where heavy shelling displaced a number of residents, the group said.

Twelve Syrian security forces were killed in an ambush by a group of defecting soldiers in the town of Dael in Daraa province, the opposition Syrian Observatory for Human Rights said.

The security forces were entering the town from the south to start a campaign of arrests when they were ambushed, the observatory said. Five of the defectors were wounded in clashes with other Syrian security forces who had arrived to evacuate the dead, the group said.

Syria, on the state-run Syrian Arab News Agency, said 17 army and law enforcement "martyrs" were buried Tuesday.

The regime got fresh backing from Russia for its assertion that terrorist groups are behind the widespread violence.

Russian Foreign Minister Sergey Lavrov, at a news conference, said that if reports of an al Qaeda presence in Syria prove true, "and I assume it is the case, then the weapons coming to Syria could fall into the hands of this terrorist organization." And he noted that, according to a U.N. Security Council resolution, all U.N. members "should not only prevent weapons from getting to al Qaeda, but must avoid all links."

Al-Assad, who has insisted he is helping lead democratic reforms, has set May 7 for parliamentary elections, the Syrian parliament website said Tuesday.

But many world leaders and opposition activists have said al-Assad is leading a violent crackdown aimed at keeping his lock on power.

The opposition Syrian National Council called Monday for urgent international military intervention to help stop the violence and to protect civilians.

The council, an umbrella group that includes opposition members abroad as well as dissidents inside Syria, also demanded a no-fly zone across the country and a "speedy operation" to arm the Free Syrian Army, rebel fighters composed primarily of defectors from al-Assad's forces.

"Sympathy messages are no longer enough. ... What is needed is actions on (the) ground and decisions and measures against (al-Assad's) gangs," the group said after a meeting in Turkey.

Yousuf, a resident and opposition activist in Homs, described incessant bombings across the city Tuesday.

"There have been over 100 explosions heard in Homs this morning caused by the continuous tank shelling and rockets" in several neighborhoods, said Yousuf, who is not being fully identified for safety reasons.

About 30,000 Syrians have fled to neighboring countries in the past year, according to Panos Moumtzis, the U.N. High Commissioner for Refugees' coordinator for Syrian refugees.

Even the flight abroad is fraught with danger, according to Human Rights Watch.

Syrian forces have placed landmines near the borders with Turkey and Lebanon, the group said Tuesday, citing witnesses and Syrians who have helped remove them.

"The Syrian army should cease its use of antipersonnel landmines and recognize that planting this internationally banned weapon will hurt Syrians for years to come," Human Rights Watch said. "Both antipersonnel and antivehicle mines of Soviet/Russian origin have been cleared by deminers associated with the opposition."

CNN cannot independently confirm reports of casualties or attacks in Syria because the government has severely restricted the access of international journalists.

But most reports from inside Syria indicate the regime is slaughtering civilians to wipe out dissidents seeking al-Assad's ouster. The al-Assad family has ruled Syria for more than four decades.