Parts of Homs are strewn with rubble and wreckage from the fighting
The Syrian army has resumed heavy shelling in the restive city of Homs as Russia's foreign minister is due in Damascus for talks.
Hundreds of people are reported to have been killed since government troops began pounding known strongholds of opposition groups.
The BBC's Paul Wood, in Homs, says residents fear troops are planning to launch a ground assault.
President Bashar al-Assad is trying to face down a months-long uprising.
Our correspondent says the Syrian army started firing mortars at about 06:00 local time (04:00 GMT). There has been fairly heavy fire, he adds.
Russia, along with China, has faced international condemnation after vetoing a UN resolution that backed an Arab League peace plan for Syria.
Russian Foreign Minister Sergei Lavrov, due to hold talks with Syrian leaders, said Western reaction to Moscow's veto had bordered on "hysteria".
His office said he was heading to Damascus because Moscow sought "the swiftest stabilisation of the situation in Syria".
US State Department spokeswoman, Victoria Nuland said: "Our hope and expectation is that foreign minister Lavrov will use this opportunity to make absolutely clear to the Assad regime how isolated it is and to encourage Assad and his people to make use of the Arab League plan and provide for a transition."
Our correspondent - one of the few foreign reporters in Homs - says that as night fell in the city, government forces continued to fire shells into residential areas.
He said residents were burying their dead in mass graves under the cover of darkness, but were still coming under fire.
Unconfirmed reports said troops had been moved up to within 1km (0.6 miles) of the parts of the city under bombardment, increasing fears of a ground attack, he adds.
Syrian authorities deny firing on houses and say security forces killed "tens of terrorists" in Homs on Monday.
UN Secretary-General Ban Ki-moon called the escalating violence in Syria "totally unacceptable before humanity", his spokesman, Martin Nesirky said.
He urged "all concerned in Syria" and the international community to redouble efforts to try to stop the violence.
The US said on Monday it had closed its embassy in Damascus and removed all remaining staff because of security concerns.
Alexander Nekrassov: "Russia is saying their diplomatic effort was undermined"
German UN Ambassador Peter Wittig said supporters of the Arab League plan - which calls for President Assad to step down - needed to explore ways forward.
He said Berlin was proposing an international contact group that he described as "a broad-based coalition of friends of the Arab League and friends of Syria above all".
"Our fears have come true," he said. "Assad used the situation once again as a licence to kill - that's the sad aftermath of the veto."
The Syrian government says it is fighting foreign-backed armed groups.
Thousands of former army soldiers have defected to the rebel side, forming the Free Syrian Army.
Human rights groups and activists say more than 7,000 people have been killed by Syrian security forces since the uprising began last March.
The UN stopped estimating the death toll in Syria after it passed 5,400 in January, saying it was too difficult to confirm.
President Assad's government says at least 2,000 members of the security forces have been killed.