* Activists say rebels holding off loyalist forces in Homs
* Hundreds of civilians under siege in old city
* Britain says Assad must halt "brutal assault"
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 their offensive in central Syria, 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, backed by guerrillas from the Lebanese Shi'ite group Hezbollah, but reported clashes and deaths within the city on Sunday.
Mohammad Mroueh, a member of the opposition "Homs Crisis Cell" said at least 25 loyalist troops including four Hezbollah fighters had been killed in Homs in the previous 24 hours. Such reports are difficult to verify in Syria, where independent media cannot usually report freely.
The offensive follows steady military gains by Assad's forces, backed by Hezbollah, in villages in Homs province and towns close to the Lebanese border.
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 an 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 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.
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.
Opposition activists said a woman and child had been killed in a strike by government aircraft on the old city of Homs, home to hundreds of civilians.
Video footage taken by the activists showed the bodies being carried in blankets and a man holding a wounded child with a gash in his head.
Rebel fighters 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."
Alawites belong to an offshoot of Shi'ite Islam and have controlled Syria since the 1960s, when members of the sect took over the army and security apparatus in the mainly Sunni country.
Homs is a majority Sunni city. But a large number of Alawites have moved there in recent decades, drawn by army and security jobs.
Lebanese security forces said Hezbollah appeared to be present in the rural areas surrounding Homs.
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.
State media said the army had "destroyed terrorist concentrations" in several districts of Homs and made "big progress" in Khalidiya.
British Foreign Secretary William Hague said Assad must halt his "brutal assault" on Homs and allow full humanitarian aid access to the country. Gulf countries, which back the rebels, urged Lebanon to stop outside parties interfering in the conflict, a reference to the Iranian-backed Hezbollah.
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.
Syria's official state news agency said a helicopter carrying Ministry of Education employees heading to a northern area to supervise school exams had been shot down. The seven employees and the helicopter's crew were killed, it said.
The agency said the plane was brought down as "part of the scheme of the armed terrorist gangs to halt normal life".
Opposition activists said the helicopter had been carrying supplies to two Shi'ite villages north of the city of Aleppo where Hezbollah fighters had been deployed.