Syria's air force bombarded Qusayr on Sunday, two weeks into a Hezbollah-backed assault, as the regime said it will only allow the Red Cross into the embattled town once it has defeated rebels.
At the United Nations, diplomats said Russia blocked a draft Security Council declaration expressing "grave concern" about the situation in the strategic town near the border with Lebanon.
Elsewhere, a car bombing killed nine security forces members in Damascus, adding to an estimated 94,000 lives lost in the more than two-year conflict, as France said a peace conference on ending the bloodshed could be delayed.
The Syrian Observatory for Human Rights said the suicide attack appeared to have been carried out by the jihadist Al-Nusra Front, although there was no confirmation.
"At least nine regime forces were killed in the explosion of a car bomb near a police station in the Jubar neighbourhood," Observatory director Rami Abdel Rahman told AFP.
State news agency SANA reported "10 citizens were wounded when a car driven by a terrorist exploded in Jubar", but gave no information on deaths.
In the central province of Homs, regime warplanes carried out multiple raids against the northern part of Qusayr and the area between the town and Dabaa, said the Observatory.
A day earlier, international aid groups called for the evacuation of civilians trapped in the town, where regime forces backed by fighters of the Lebanese Shiite movement Hezbollah launched an all-out offensive on March 19.
But Syria's Foreign Minister Walid Muallem told UN chief Ban Ki-moon in a telephone call that the Red Cross would not be allowed to enter until the assault is over.
"Syrian authorities will allow the Red Cross in cooperation with the Syrian Red Crescent access to the area immediately after the end of military operations," SANA quoted him as saying.
The Observatory said the regime has continued to bolster its forces in Qusayr, key to the regime and the rebels alike as it links Damascus to the coast, and is near the Lebanese border, providing a conduit for weapons and fighters.
On Saturday, Ban and international aid groups expressed concern about civilians trapped in Qusayr, and for between 1,000-1,500 injured residents still in the town of 25,000 people.
But Muallem voiced "surprise" at the concern over Qusayr, SANA reported, "given that no one expressed this concern when terrorists took control of the city and the surrounding area."
And at the UN, Syria's most powerful ally Russia demanded "wider political discussion" after blocking a Security Council declaration that expressed concern about civilians trapped in Qusayr, diplomats said.
The international community has pinned its hopes for resolving the conflict peacefully on a US-Russian initiative to hold a conference that had been mooted for June in Geneva.
But French Foreign Minister Laurent Fabius said it could be delayed.
"'Geneva 2' is in my opinion a last-chance conference. I hope it will take place, I think it could take place in July," he said.
The fighting in Syria has increasingly drawn in neighbouring Lebanon, despite Beirut's official neutrality on the conflict.
Members of Hezbollah, a staunch ally of President Bashar al-Assad's regime, are fighting alongside government troops.
And some members of Lebanon's Sunni community have also crossed the border to fight alongside the Sunni-led rebels, encouraged by clerics including the influential Qatar-based Yusuf al-Qaradawi.
On Sunday, a security source said Hezbollah fighters and rebels clashed just inside Syria along the Lebanese border, leaving one Hezbollah member dead.
The fighting also spilled into Lebanon, with two rockets fired from Syria landing in the northeastern region of Hermel, a Hezbollah stronghold.
The six-member Gulf Cooperation Council, meeting in the Saudi city of Jeddah, warned it could take measures against interests of Hezbollah.
It gave no other details, but Bahraini Minister of State for Foreign Affairs Ghanim al-Buainain said that "nobody could cover up Hezbollah's actions in regional countries.
"It is a terrorist organisation and this is how Gulf states see it," he added.
Lebanon's President Michel Sleiman, meanwhile, said his country would file an "urgent complaint" to the United Nations after Israeli warplanes violated Lebanese airspace.
AFP correspondents across Lebanon, including Beirut, reported hearing the planes flying overhead.