FILE - In a Tuesday, June 5, 2012 file photo, an elderly Syrian man stands on the side of a street in the town of Taftanaz, 15 kilometers east of Idlib, Syria. At dawn on April 3, Syrian forces shelled the town in the first volley of what residents say was a massive assault after a string of large protests calling for the end of the autocratic rule of President Bashar Assad. (AP Photo/Khalil Hamra, File)

BEIRUT - Syrian forces renewed shelling of the central city of Homs on Monday, one day after the head of the U.N. observers' mission demanded that warring parties allow the evacuation of women, children, elderly and sick people, activists said.

Regime forces have been waging a fierce offensive through towns and villages nationwide, trying to root out rebels by shelling urban areas with tanks and attacking from helicopters. Rebels also have attacked Syrian forces, mostly trying to burn tanks.

"There is renewed shelling and shooting in the city of Homs," said the Britain-based Syrian Observatory for Human Rights, an activist group that cited witnesses on the ground.

Gunfire and explosions also were reported in Homs, the Observatory said.

Opposition groups say more than 14,000 people have been killed since the Syrian uprising began in March 2011 with mostly peaceful protests against President Bashar Assad's autocratic regime. But a ferocious government crackdown led many to take up arms, and the conflict is now an armed insurgency.

The Observatory says more than 3,400 soldiers and militiamen loyal to the government have also been killed since the revolt began.

On Sunday, the head of the U.N. observers' mission in Syria Maj. Gen. Robert Mood said the monitors had been trying for the past week to bring out families and wounded trapped in Homs by heavily shelling of rebel-held areas. The offensive is part of the broader push by Assad's forces to regain rebel-held villages and towns throughout the country.

"The parties must reconsider their position and allow women, children, the elderly and the injured to leave conflict zones without any preconditions and ensure their safety," Mood said in a statement. U.N. "attempts to extract civilians from the line of fire over the past week have been unsuccessful."

"This requires willingness on both sides to respect and protect the human life of the Syrian people," he added.

On Saturday, the U.N. said its 300 observers based in Syria were suspending all missions because of concerns for their safety after fighting intensified over the previous 10 days. But the monitors said they were remaining in Syria in Damascus.

The conflict in Syria has brought broad international condemnation, but the regime has appeared largely impervious to the pressure.

U.S. Sen. John McCain on Sunday characterized the lack of American aid to Syrian rebels as "shameful" and said helping their cause would deal "the greatest blow to Iran in the Middle East in 25 years." His remarks sought to maintain political pressure on President Barack Obama as violence in the region escalated.

"The fact that the United States of America is not helping these people — and we can — is shameful," McCain, who ran against Obama in 2008, said Sunday on NBC's "Meet the Press."

So far, the U.S. has refused to arm Syrian rebels in part to avoid a proxy fight with Iran and Russia, which both back Assad's regime.

News from © The Associated Press, 2012