President Bashar al-Assad's forces pounded Sunni Muslim rebels in the city of Homs with artillery and from the air on Sunday, the second day of an offensive to expand loyalist control over Syria's strategic centre, activists said.

They said rebels defending the old centre of Homs and five adjacent Sunni districts had largely repelled a ground attack on Saturday by Assad's forces but reported fresh clashes and deaths within the city on Sunday.

The offensive follows steady military gains by Assad's forces, backed by Lebanese Hezbollah militants, in villages in Homs province and towns close to the Lebanese border.

British Foreign Secretary William Hague said Assad must halt his "brutal assault" on Homs. Gulf countries, which back the rebels, urged Lebanon to stop "parties" interfering in the Syria conflict, a reference to Iranian-backed Hezbollah.

Opposition sources and diplomats said the loyalist advance had tightened the siege of Homs and secured a main road link to Hezbollah strongholds in Lebanon and to army bases in Alawite-held territory near the Syrian coast, the main entry point for Russian arms that have given Assad a key advantage in firepower.

At least 100,000 people have been killed since the Syrian revolt against four decades of rule by Assad and his late father erupted in March 2011, making the uprising the bloodiest of the Arab Spring popular revolutions against entrenched autocrats.

The Syrian conflict is increasingly pitting Assad's Alawite minority, backed by Shi'ite Iran and its Hezbollah ally, against mainly Sunni rebel brigades supported by the Gulf states, Egypt, Turkey and others.

Sunni Jihadists, including al Qaeda fighters from Iraq, have also entered the fray.

ALARM

The loyalist advances have alarmed international supporters of the rebels, leading the United States to announce it will step up military support. Saudi Arabia has accelerated deliveries of sophisticated weaponry, Gulf sources say.

The Sham News Network opposition monitoring group said fighters belonging to the al Qaeda-linked Nusra Front had killed five loyalist troops in fighting in the Bab Hud district of Old Homs on Sunday.

Activists said one woman and a child had been killed in an airstrike on the old city, home to hundreds of civilians.

Video footage taken by the activists, which could not be immediately verified, showed the two bodies being carried in blankets as well as a man holding a wounded child with a huge gash in his head.

Rebel fighters also fought loyalist forces backed by tanks in the old covered market, which links the old city with Khalidiya, a district inhabited by members of tribes who have been at the forefront of the armed insurgency.

"After failing to make any significant advances yesterday, the regime is trying to sever the link between Khalidiya and the old city," Abu Bilal, one of the activists, said from Homs.

"We are seeing a sectarian attack on Homs par excellence, The army has taken a back role. Most of the attacking forces are comprised of Alawite militia being directed by Hezbollah."

The Alawites are an offshoot of Shi'ite Islam that have controlled Syria since the 1960s, when members of the sect took over the army and the security apparatus which underpin the power structure in the mainly Sunni country.

URBAN WARFARE

Located at a major highway intersection 88 miles north of Damascus, Homs is a majority Sunni city. But a large number of Alawites have moved into mostly new and segregated districts in recent decades, drawn by army and security jobs.

Lebanese security forces said Hezbollah appeared to be present in the rural areas surrounding Homs but there was no indication that it was fighting in the labyrinth streets of Homs, where it could take heavy casualties.

Anwar Abu al-Waleed, an activist, said rebel brigades were prepared to fight a long battle, unlike in Qusair and Tel Kalakh, two towns in rural Homs near the border with Lebanon that fell to loyalist forces in recent weeks.

"We are talking about serious urban warfare in Homs. We are not talking about scattered buildings in an isolated town but a large urban area that provides a lot of cover," he said.

Britain's Hague expressed concern over the escalation of fighting in Homs, saying in a statement: "I call upon the Assad regime to cease its brutal assault on Homs and to allow full humanitarian access to the country."

The Syrian conflict has aggravated neighbouring Lebanon's own complex sectarian rivalry, triggering fighting between Alawite pro-Assad and Sunni anti-Assad militia in the northern city of Tripoli that has killed dozens.

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Gulf foreign ministers meeting in Bahrain urged the Lebanese government to "commit to distancing itself from the Syrian crisis and to prevent any Lebanese parties from interfering in (Syria) in order to enable it to confront the brutal attacks and crimes conducted by the regime and its allies."

Additional reporting by Angus McDowall and William Maclean in Dubai