Columns of Syrian tanks descended on Hama on Tuesday in what besieged residents feared would be an escalation of a three-day assault on a city at the heart of the Syrian uprising.

The offensive, which has killed at least 80 people in Hama since Sunday, according to rights groups, drew international outrage and spurred a long-stalled effort at the United Nations to condemn Syria’s attacks against civilians.

In Washington, meanwhile, Secretary of State Hillary Rodham Clinton met with Syrian activists to “express our solidarity with the Syrian people,” said State Department spokesman Mark Toner. He said U.S. officials are hammering out new sanctions to further isolate the government of President Bashar al-Assad.

Nearly 2,000 Syrians have been killed in the four-month-old rebellion, and the violence “is getting worse,” the U.S. ambassador to Syria, Robert Ford, said in Senate testimony Tuesday. “The Syrian government’s constant brutality, its refusal to allow peaceful marches, its insistence on widespread arrest campaigns and its atrocious torture . . . they are, in turn, fostering more violence,” the diplomat said.

In Hama, columns of tanks were seen converging at the southern approach to the city throughout the day, according to activist Saleh Hamawi. After nightfall, families huddled indoors as explosions echoed through the deserted streets. It appeared to be a repeat of the previous night’s assault, during which tanks began shelling the city as citizens gathered for the meal breaking their daylong Ramadan fast, residents said.

“There are a lot of reinforcements, and we fear they are planning a big operation,” Hamawi said, speaking by telephone from Hama. “We feel another massacre is coming. There’s nothing we can do to stop it, and the international community doesn’t care.”

Another resident, who asked to be identified by the nickname Abu Zaid, said residents feared that they would soon run out of food because stores were shuttered and people were afraid to leave their homes because of the threat of snipers.

Though some residents fled the city Tuesday, most did not dare because of the tight security cordon, he said.

“Each day is getting worse than the day before, and we cannot do anything,” Abu Zaid said.

The Local Coordinating Committees, a group that monitors and organizes protests, said five people died Tuesday in Hama, the site of the 1982 massacre of at least 10,000 residents by forces loyal to then-President Hafez al-Assad, father of the current leader.

The latest deaths brought to at least 130 the number of people killed across Syria since the government launched its offensive Sunday, the eve of the Muslim holy month of Ramadan.

There has been no sign, however, that the brutality is deterring the opposition, and anger at the bloodshed in Hama may have only served to intensify demonstrations. Since Sunday, protesters have taken to the streets in numerous locations across Syria after daily evening prayers.

At the United Nations, the Security Council held a closed-door meeting to review a resolution — drafted by European countries and supported by Washington — that would condemn Syria’s attacks on protesters.

The resolution, originally introduced in May, had stalled because of opposition from Russia and China. But on Tuesday, a senior Russian Foreign Ministry official told news agencies that his country was “not categorically against” a resolution, as long as it did not impose sanctions.

In Washington, Ford told senators that the Syrian government’s crackdown in places such as Hama and the eastern city of Deir el-Zour “had an impact,” adding that countries that had opposed Security Council action “are potentially now more open to some kind of action.”

U.S.-based leaders of the Syrian opposition movement met with Clinton at the State Department on Tuesday, the first time they had been granted such high-level access. During the hour-long discussion, they pressed for stronger international measures against the Syrian regime.

“It is clear that Clinton is fully aware of what is happening in Syria and is looking for a way to help us,” said Radwan Ziadeh, a human rights activist who fled Syria in 2007.

The six activists called for stronger U.S. leadership on oil, gas and weapons sanctions against the regime. They also asked the administration to call for Bashar al-Assad to leave office and for the International Criminal Court to charge him with crimes against humanity. They said Clinton had expressed concern that the Syrian government could try to portray such actions as evidence that the uprising was American-sponsored.

Ziadeh said the activists urged Ford to remain in Damascus and keep up links with the opposition.

Italy recalled its ambassador to Syria on Tuesday to protest the Damascus government’s crackdown, but it did not appear that other European countries would follow suit.

Also Tuesday, three senators said they would introduce a bill to penalize companies investing in Syria’s energy sector or selling gasoline to the country. Such a measure would mostly affect Canadian, European and Chinese energy companies.

The measure, announced by Sens. Joseph I. Lieberman (I-Conn.), Kirsten Gillibrand (D-N.Y.) and Mark Kirk (R-Ill.), would extend existing sanctions to foreign companies.

Toner told reporters that the U.S. government planned “to move forward with additional sanctions under existing authorities, and we’re exploring the scope of those sanctions.”

Sheridan reported from Washington. Staff writer Alice Fordham in Washington contributed to this report.