BEIRUT – Rebels brazenly battled government security forces in the heart of Damascus for the first time on Friday, witnesses said, and explosions echoed for hours.

Elsewhere, government artillery repeatedly pounded the central city of Homs and troops tried to storm it from three sides, and U.N. observers saw body parts scattered around a Syrian farming hamlet in Hama that was the scene of a massacre this week in which nearly 80 men, women and children were reported slain.

In Damascus, government troops clashed with defectors from the Free Syrian Army in the Kfar Souseh district in some of the worst fighting yet in the capital. The clashes were a clear sign that the ragtag rebel group has succeeded in taking its fight into the regime’s base of power.

“I’ve been hearing shooting and explosions for hours now and can see smoke rising from the area,” a witness who spoke on condition of anonymity for security concerns told The Associated Press.

Friday’s fighting began when the rebels attacked a government checkpoint in the morning, according to Rami Abdul-Rahman of the Syrian Observatory for Human Rights in Great Britain.

A resident of the Damascus neighborhood of Qaboun said the battles began in his area after Syrian forces opened fire on a demonstration, killing a young man he identified as Mahmoud Said. Following that, gunmen hiding in the area began clashing with security forces. Nobody was sure how many people were killed, because they could not leave their houses, said the resident, who asked not to be identified because he feared government reprisal.

The Observatory and another activist group, the Local Coordination Committees, said clashes also broke out in other Damascus districts. There was no word on civilian casualties, but the LCC said three rebels were killed.

Killings in Hama

Observers were finally able to get inside the deserted village of Mazraat al-Qubair on Friday after being blocked by government troops and residents and coming under small-arms fire Thursday, a day after the slayings were first reported.

The U.N. team was the first independent group to arrive there, a village of about 160 people in central Hama province. Opposition activists and Syrian government officials blamed each other for the killings and differed about the number of dead.

Sausan Ghosheh, spokeswoman for the U.N. observers, said Mazraat al-Qubair was “empty of the local inhabitants.”

“You can smell the burnt smell of the dead bodies,” Ghosheh said. “You could also see body parts in and around the village.”

She told the BBC: “We can say that there was definitely a horrific crime that was committed. The scale is still not clear to me.”

A BBC correspondent traveling with the U.N. observers described the hamlet as an “appalling scene” of burned-out houses and gore.

“There are pieces of human flesh lying around the room, there is a big pile of congealed blood in the corner, there’s a tablecloth that still has the pieces of someone’s brain attached to the side of it,” said the correspondent, Paul Danahar.

The U.N. observers also visited a cemetery where some of the dead were buried, according to an activist in Mazraat al-Qubair.

Activists said the Sunni hamlet is surrounded by Alawite villages. Alawites are an offshoot of Shiite Islam and Assad is a member of the sect, while the opposition is dominated by Sunnis.

The United States condemned Assad over the killings, saying he has “doubled down on his brutality and duplicity.”