Syrian government forces backed by helicopter gunships battled rebels in heavy clashes in Damascus, a clear escalation in the most serious fighting in the capital since the country's conflict began last year, activists said.

The fierce clashes, which have raged over the past three days in at least four neighborhoods across the city, were the latest sign that Syria's civil war is moving ever closer to the heart of President Bashar Assad's regime. Government forces have already thrown tanks and armored personnel carriers into the battle in the capital, but the use of airpower reflected the intensity and seriousness of the fighting.

The Britain-based Syrian Observatory for Human Rights and Damascus-based activist Maath al-Shami said Tuesday's clashes were concentrated in the neighborhoods of Kfar Souseh, Nahr Aisha, Midan and Qadam.

"I can hear cracks of gunfire and some explosions from the direction of Midan," al-Shami told The Associated Press via Skype. "Black smoke is billowing from the area."

Syria's state-run news agency said troops were still chasing "terrorist elements" who had fled from Nahr Aisha to Midan. Syria refers to its opponents as terrorists.

An amateur video showed two armored personnel carriers with heavy machineguns on top along with troops who were said to be advancing in an empty road toward Midan on Tuesday.

Activists have dubbed the fighting in the capital the "Damascus Volcano" in what appears to be an attempt to bring the fighting into Syria's seat of power.

The clashes are the most sustained and widespread in the capital since the start of the uprising against Assad in March last year and a crackdown that activists say has claimed the lives of more than 17,000 people. In the past, clashes happened at night in the capital. Now, the fighting rages during the day — a sign of the growing strength and boldness of the rebels.

That has brought the bloodshed to the heart of Damascus — and Syria's largest city, Aleppo — which are both home to elites who have benefited from close ties to Assad's regime, as well as merchant classes and minority groups who worry their status will suffer if Assad falls.

Activists also reported clashes and shelling in the rebel held towns of Rastan and Talbiseh in the central province of Homs.

One amateur video posted online showed helicopters firing missiles in Talbiseh, while another video showed gunmen attacking what appeared to be a government position in Rastan. The caption said the rebels were carrying "intensive operations" in support of Damascus and Homs.

As the violence across the country has spiraled out of control, diplomatic efforts to halt the bloodshed have seemingly come to a dead end.

Much of the international community has condemned Assad's crackdown, but world powers remain deeply divided over who is responsible and how to stop the fighting. The U.S. and many Western nations have called on Assad to leave power, while Russia, China and Iran have stood by the regime.

UN chief Ban Ki-moon was headed to China on Tuesday as part of a diplomatic push to get Beijing and Moscow to back a tougher response to attacks by Assad's regime. Ban's trip comes ahead of a UN Security Council vote this week. A Western-backed resolution calls for sanctions and invokes Chapter 7 of the UN Charter.

A Chapter 7 resolution authorizes actions that can ultimately include the use of military force, which US administration and European officials — for now — are playing down as a possibility.

Ban was to hold talks with Chinese President Hu Jintao on Wednesday, with Syria expected to top the agenda.

UN special envoy Kofi Annan, meanwhile, was meeting with Russian President Vladimir Putin in Moscow to discuss the conflict. Annan is seeking support for his faltering peace plan, which put some 300 UN observers in Syria to monitor the implementation of a cease-fire.

That truce never took hold, however, and the monitors have been largely confined to their hotel for weeks because of the escalating bloodshed. The mission expires on July 20.

The leader of Syria's outlawed Muslim Brotherhood, Mohamad Riyad Shukfa, criticized Annan's six-point plan and the observer mission, saying "the Syrian people have no faith in it."

"We would like this mission to leave Syria soon because it gives more chances to Assad to kill," Shukfa said from Turkey, where the Brotherhood, which was crushed in 1982 by Assad's late father and predecessor, Hafez Assad, is holding a meeting.

In Tehran, Iranian Foreign Ministry spokesman Ramin Mehmanparast warned against foreign intervention in Syria. Iran, one of Syria's strongest allies, offered to mediate and host a meeting between Syrian government and rebels to help resolve the conflict.

"Iran is prepared to use all its capacities to resolve the crisis in Syria," Mehmanparast told reporters Tuesday.

The Syrian opposition is unlikely to accept Iran, which has provided Assad with military and political backing for years, as a mediator.

The Observatory and al-Shami said helicopters fired heavy machine guns during overnight clashes in the neighborhoods of Qadam and Hajar al-Aswad.

An amateur video showed a military helicopter flying over the Damascus neighborhood of Qaboun. The narrator could be heard saying "aerial bombardment" of Qaboun, although the helicopter was not seen firing in the 30-second video. The authenticity of the video could not be independently verified.

Another activist in the capital, who asked to be identified only as Bayan because of fear for her safety, said troops fired mortar rounds at the neighborhoods of Qaboun and Jobar, causing a major fire near Jobar's Grand Mosque.

"People are trying to extinguish the fire with water hoses or buckets filled with water," Bayan said via Skype. She added that government troops had set up checkpoints around Damascus and were searching cars and demanding passengers' identity cards.

Al-Shami said residents of hard-hit areas were fleeing to safer neighborhoods where they were staying at schools and mosques. He added that many of the wounded were being treated at secret hospitals for fear they might be detained if taken to official ones.

The Local Coordination Committees activist group reported heavy shelling of rebel-held areas in the central province of Homs where they said there were casualties.

As the violence has escalated, the number of Syrians fleeing the bloodshed has swelled to some 112,000, according to United Nations. The U.N. refugee agency said women and children make up three-quarters of those whom it has assisted or registered in Iraq, Jordan, Lebanon and Turkey.##