CAIRO — Two Iranian warships docked in a Syrian port on Monday as a senior Iranian lawmaker denounced American calls for arming the Syrian opposition, adding to the international tensions over the nearly yearlong crackdown by the government of President Bashar al- Assad.

As government forces continued to pound opposition strongholds, the International Committee of the Red Cross said it was trying to negotiate a brief pause in the violence to deliver aid to the most devastated areas.

Activist groups reported intensified attacks on the besieged Baba Amr neighborhood in the central city of Homs. They said the government’s inability to eradicate the opposition there despite weeks of bombardment could be preventing the military from striking deeper and harder into other parts of the country where armed resistance and rebellion are believed to be growing, including Hama and Idlib Province to the north.

“The biggest challenge in Homs is Baba Amr,” said Wissam Tarif, of the activist group Avaaz. “They cannot move military power to Idlib or Hama without finishing Homs first. They cannot leave any pockets of resistance behind them.” He said 16 people were killed in Homs on Monday. Such reports are impossible to verify.

A video posted on YouTube showed an artillery strike on Baba Amr that sent a plume of dark smoke into the clear, sunny sky. “God is my only and best guardian,” muttered the panicked videographer. “The world remains silent. Today is Feb. 20, 2012.”

Armed rebels have provided a measure of security to some protesters in places like Hama, which was leveled 30 years ago as Mr. Assad’s father, Hafez al-Assad, put down an uprising by the Muslim Brotherhood, killing at least 10,000 people. In a video posted on YouTube on Monday, several hundred people jumped and danced in Halfaya, a neighborhood of Hama, in what the video described as a regular “morning protest.”

Still, more than 50 checkpoints divide up the city, said the Local Coordination Committees, a grass-roots group that organizes and documents protests, and security forces have detained more than 500 people there in the past three weeks.

The living situation in Hama, Homs and other hard-hit areas, including two suburbs of Damascus, Zabadani and Madaya, has become increasingly grim. In Homs, supplies of food, baby formula, medicine and potable water are all running out, said a spokeswoman for the committees.

“About two weeks ago, we sent 200 cans of baby milk into Homs, and they said they could not even meet 10 percent of their needs,” said the spokeswoman, Jasmine, who asked to use only one name for security reasons. “Now, no one can get in or go out.”

The International Committee of the Red Cross said it had begun negotiations with Syrian authorities for a pause in the fighting of as little as a few hours.

“The I.C.R.C. is exploring several possibilities for delivering urgently needed humanitarian aid,” Reuters quoted a spokeswoman, Carla Haddad, as saying. “These include a cessation of fighting in the most affected areas to facilitate swift Syrian Arab Red Crescent and I.C.R.C. access to the people in need.”

The Iranian ships arrived in the Syrian port, Tartus, days after the sharpest international rebuke to Mr. Assad so far: the passage of a resolution in the United Nations General Assembly condemning the crackdown and calling for him to step aside. The current escalation of attacks on Homs and other areas began early this month, after the same resolution was vetoed in the Security Council by Russia and China. Russia recently sent ships to the same Syrian port, activists said. Iran’s semiofficial Fars News Agency called the ships “a serious warning” to the United States, and quoted a senior Iranian lawmaker’s denunciations of comments by Senator John McCain a day earlier in support of arming the Syrian opposition.

“The presence of Iran and Russia’s flotillas along the Syrian coast has a clear message against the United States’ possible adventurism,” said Hossein Ebrahimi, a vice chairman of the Iranian Parliament’s national security and foreign policy commission, Fars reported.

The mission and cargo of the ships are unknown.

“In case of any U.S. strategic mistake in Syria, there is a possibility that Iran, Russia and a number of other countries will give a crushing response to the U.S.,” said Mr. Ebrahimi, according to Fars.

Mr. McCain, an Arizona Republican who was in Afghanistan on Sunday, told reporters there that he was in favor of arming the Syrian opposition, while stressing that no direct American involvement was necessary. In Cairo on Monday, he repeated that position.

“I am not calling for direct supply of weapons to Syria,” Mr. McCain said. “We have seen in Libya that there are ways to get weapons to people so that they can defend themselves. It is time that we gave them the wherewithal to fight back and stop the slaughter.” Rod Nordland contributed reporting from Cairo. (Copyright 2012 The New York Times Company 02/21/12)