Russian Foreign Minister Sergei Lavrov is in Syria for talks with President Bashar al-Assad amid a renewed government assault on rebel forces.
Mr Lavrov was greeted by pro-Assad crowds as he arrived in Damascus. The visit comes after Russia and China vetoed a Western-backed UN resolution criticising the crackdown in Syria.
Turkey has said it will start a new international initiative on Syria.
The Syrian army has been pounding the rebel stronghold of Homs for days.
The BBC's Paul Wood - one of the only foreign reporters in Homs - says the Syrian army started firing artillery at about 06:00 local time (04:00 GMT) on Tuesday.
He says residents fear troops are planning to launch a ground assault.
Hundreds are reported to have died since the heavy shelling began on Friday. At least 95 people were killed on Monday alone, activists say.
Cut off
Thousands of President Assad's supporters lined the streets of Damascus and waved flags as Mr Lavrov's motorcade drove through the city.
On Monday, Mr Lavrov said Western reaction condemning Russia's veto of the UN Security Council resolution on Saturday bordered on "hysteria".
Moscow has said the draft - which backed an Arab League peace plan calling for President Assad to hand over power - would have forced regime change on Syria.
Mr Lavrov's office said he was heading to Damascus because Moscow sought "the swiftest stabilisation of the situation in Syria".
US State Department spokeswoman Victoria Nuland urged Mr Lavrov to "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".
Russia is the main supplier of arms to Damascus. The Syrian port of Tartus is home to Russia's only Mediterranean naval base.
Meanwhile Turkish Prime Minister Recep Tayyip Erdogan said Ankara would start a new diplomatic initiative with countries opposed to Syria, following the "UN fiasco".
The Syrian government, which has been fighting an uprising against President Assad's rule since March, says it is fighting foreign-backed armed gangs.
Thousands of former army soldiers have defected to the rebel side, forming the Free Syrian Army.
Syria's interior ministry said operations against "terrorist groups" would continue until "security and order are restored" in Homs.
Homs activist Mohammed al-Hassan told Reuters news agency that Tuesday's bombardment was mostly concentrated on the restive Baba Amr district.
"There is no electricity and all communications with the neighbourhood has been cut," he added.
'Licence to kill'
Meanwhile, the president's UK-born wife, Asma Assad, has expressed her support for her husband.
An email sent to Britain's Times newspaper from her office said: "The president is the president of Syria, not a faction of Syrians, and the first lady supports him in that role."
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.
German UN Ambassador Peter Wittig said supporters of the Arab League plan 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."
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.
