BEIRUT (AP) — Syrian troops on Friday heavily shelled a rebel-held neighborhood in the flashpoint central city of Homs in a fierce assault as the military appeared to be readying to storm the district, activists said.

Also, U.N. observers entered a remote farming area where a massacre was reported this week, an activist said, a day after they were blocked from reaching it by troops and local residents and fired upon.

In Homs, one of the main battlegrounds in the 15-month-old uprising against President Bashar Assad, regime troops were trying to advance into the opposition-held district of Khaldiyeh from three sides, battling with armed rebels trying to stop them, said Tarek Badrakhan, an activist based in the neighborhood speaking via Skype.

“This is the worst shelling we’ve had since the start of the revolution,” he said. A shell could be heard exploding in the background as he spoke.

Shells were hitting the neighborhood at a rate of five to 10 a minute, said the Britain-based Syrian Observatory for Human Rights. “It seems they are trying to enter it today,” it said.

The Observatory and another activist group, the Local Coordination Committees, had no immediate word on casualties. Amateur videos posted online showed a small white plane, apparently a drone, flying over Homs.

Amateur videos showed missiles exploding into balls of flames in the crowded concrete jumble of homes, with thundering crashes that sent plumes of heavy gray smoke over Homs. The videos suggested the attack began at dawn, as birds chirped, roosters crowed, and the sun cast a yellow glow. In one video, the missiles came in rapid succession, four exploding in less than a minute.

Homs has been one of the hardest hit regions in Syria since the start of the uprising. In April, the U.N. said more than 9,000 people have been killed since the crisis began, but it has been unable to update its estimate since and the daily bloodshed has continued in past weeks. Activists put the number of dead at about 13,000.

The reported massacre this week took place in the nearby province of Hama, where about 80 people, including women and children, were shot or stabbed Wednesday in the small farming areas of Mazraat al-Qubair.

A group of U.N. observers entered the area on Friday, visiting a cemetery where some of the dead were buried and then continuing to the site of the killings, according to Leith Al-Hamwy, an activist in Mazraat al-Qubair.

A team that tried to reach the area on Thursday was blocked by troops and local residents and then fired on.

The reports of the killings there came in the wake of another mass killing in a string of villages known as Houla, where 100 people were killed last month. The opposition and the regime blamed each other for the Houla massacre.

As reports emerged about Mazraat al-Qubair, which would be the fourth such mass killing of civilians in Syria in the last two weeks, the United States condemned Assad, saying he has “doubled down on his brutality and duplicity.”

The government denied responsibility. A government statement Thursday on the state-run news agency SANA said “an armed terrorist group committed an appalling crime” in Mazraat al-Qubair, killing nine women and children. It said residents appealed for protection from Hama authorities, who sent security forces who went to the farm, stormed a hideout of the group and clashed with its fighters.

In Geneva, International Committee of the Red Cross spokesman Hicham Hassan told reporters Friday that the humanitarian situation in Syria was worsening.

“Currently the situation is extremely tense, not only in Houla, not only in Hama, but in many, many places around the country,” he said. He noted the countryside around the northern city of Idlib, suburbs of the capital Damascus, the eastern province of Deir el-Zour and the coastal region of Latakia had all seen attacks.

On Friday, troops fired tear gas and live ammunition in several locations across the country in an attempt to disperse thousands of anti-government protesters, activists said, including the northern provinces of Idlib and Aleppo, the southern region of Daraa and the suburbs of the capital, Damascus. There was no immediate word on casualties.

Syria’s state-run media said armed “terrorist groups” attacked military units charged with protecting al-Omar oil field of al-Furat Oil Company in the oil-rich city of Deir Ezzor province. The official news agency SANA said several gunmen were killed in the Friday attack.

SANA also said a car bomb in the Damascus suburb of Qudsaya killed three policemen, while another explosion in the northern city of Idlib killed two soldiers and three civilians.

In Brussels, Kristalina Georgieva, European commissioner for humanitarian aid, said there are 1 million “vulnerable people who need humanitarian assistance” in Syria.

“Between 200,000 and 400,000 are internally displaced … and we have 95,000 refugees in Turkey, Lebanon and Jordan primarily,” she said.

U.N. patrols in Syria have on several instances been deliberately targeted with heavy weapons, armor-piercing ammunition and a surveillance drone, U.N. Secretary-General Ban Ki-moon told the Security Council on Thursday, according to a senior U.N. official. The official, speaking on condition of anonymity because the council meeting was closed, said Ban also reported repeated incidents of firing close to U.N. patrols, apparently to get them to withdraw.

International envoy Kofi Annan, whose peace plan brokered in April has not been implemented, warned against allowing “mass killings to become part of everyday reality in Syria.”

“If things do not change, the future is likely to be one of brutal repression, massacres, sectarian violence, and even all-out civil war,” Annan told the U.N. General Assembly in New York. “All Syrians will lose.”

U.N. diplomats said Annan was proposing that world powers and key regional players, including Iran, come up with a new strategy to end the 15-month conflict.

___

AP writers Frank Jordans in Geneva and Slobodan Lekic in Brussels contributed to this report.

Copyright 2012 The Associated Press.