An ominous communications blackout descended over the besieged Bab Amr district in the central Syrian city of Homs on Wednesday as Syrian troops, backed by tanks, launched what appeared to be a major offensive aimed at wresting back control of the area from government opponents.

There were reports of at least one fierce battle on the outskirts of the neighborhood, and residents elsewhere in Homs described intense shelling there and in many other parts of the city. There were fears that the violence marked the start of an all-out attempt to crush resistance in the epicenter of the nearly year-long revolt against Syrian President Bashar al-Assad.

But with almost all communications to Bab Amr severed, including the satellite phones that activists have been using to transmit news and images of the fighting to the outside world, it was difficult to establish exactly what was happening.

Some activists said the government had blocked satellite transmissions; others said fuel supplies had run out for the generators that besieged residents use to power laptops and satellite phones. In the first instance of its kind, opposition groups said they could not provide casualty figures for the day’s fighting in Homs because they could not reach their contacts.

Activists in other parts of Homs described widespread fear as explosions echoed across the city. One activist, who identified himself only as Abu Emad, said government ground forces tried to enter the Bab Amr neighborhood Wednesday morning near a sports stadium to the northeast but ran into stiff resistance from rebels of the Free Syrian Army.

“A lot of soldiers tried to enter, but they couldn’t until now because the Free Syrian Army is fighting back,” said the activist, speaking over a satellite connection.

Many other neighborhoods also were being bombarded by tank and shell fire, Abu Emad said, adding: “The situation is catastrophic.”

A defiant statement, purportedly issued by the Free Syrian Army and posted on a Facebook page run by a Bab Amr activist who is now outside the country, said members of the elite 4th Armored Division commanded by Maher al-Assad, the president’s brother, had been held at bay after the soldiers attempted to enter the neighborhood.

“The Assad army did not enter Bab Amr at all, and they will never enter Bab Amr, God willing, with the brave Free Syrian Army’s protection,” the statement said. Government assertions that the rebel fighters are exhausted are baseless, it added. “This is all lies and has no truth to it. We will continue until the end.”

But an activist hiding in Homs’s Old City said he thought it unlikely that the Free Syrian Army was capable of resisting a major offensive. Mulham Jundi, a member of the opposition Syrian National Council who sneaked into Homs nearly two weeks ago, said that many rebel fighters had fled Bab Amr and that those who remain are running out of ammunition. He said he left Bab Amr this week after being hit in the leg by a bullet.

With food, water, medicine and fuel running out, Jundi said, the Syrian army intends to starve and freeze the residents into submission. Snow is falling in Homs, further burdening residents.

“This is their strategic aim,” he said. “The Assad forces just want to kill all the people from outside. They have no medical supplies, and they are out of food, and I’m not sure if they can survive for very much longer.”

In Damascus, an unidentified Syrian official indicated that an offensive was underway, telling the Associated Press that Homs would be “cleaned” within hours.

The intensified assault came 26 days after troops began shelling the neighborhood, which has become a symbol of resistance to Assad’s rule. Hundreds are thought to have died in the daily bombardments of the residential area, which had fallen under the control of Free Syrian Army fighters over months of clashes.

In Washington, the State Department summoned Syria’s top diplomat, Charge d’Affaires Zuheir Jabbour, to “express our outrage over the month-long campaign of brutality and indiscriminate shelling” of Homs, according to a department statement.

Two French journalists remain trapped in Homs, along with at least 20,000 civilians, but Spanish reporter Javier Espinosa was smuggled to safety in Lebanon on Wednesday, his employers said.

A British photographer escaped to Lebanon on Tuesday despite an ambush that killed 13 Syrian members of his rescue party.

Britain’s foreign secretary William Hague said Thursday that London is closing its embassy in Damascus and withdrawing all diplomatic staff in light of the worsening security situation in the country.

The U.N. Security Council, meanwhile, met to consider a statement calling on Syria to cooperate with the United Nations and demanding that it provide “full and unimpeded humanitarian access’’ to needy civilians. The top U.N. relief official, Valerie Amos, protested Syria’s refusal to allow her into the country.

Kofi Annan, the U.N.-Arab League envoy for Syria, urged Assad to invite him to Damascus and to “engage” in talks about the future of the country. Speaking alongside U.N. Secretary General Ban Ki-moon on Wednesday, Annan urged countries to rally behind his efforts to mediate an end to the crisis, saying that when the international community “speaks with one voice, that voice is power.”

In Geneva on Thursday, the U.N. Human Rights Council voted on a Turkey-sponsored resolution condemning Syria for its “widespread and systematic violations” against civilians in its crackdown.

Secretary of State Hillary Rodham Clinton said at a House hearing on Wednesday that “we are absolutely determined that we’re going to find ways forward to bring relief to the Syrian people, to support a legitimate opposition that is inclusive, that doesn’t leave out any aspect of the Syrian society and that continues to push for an end to the Assad regime.”

Staff writers Karen DeYoung in Washington and Colum Lynch in New York contributed to this report.