The UN has expressed concern about the situation in the southern Syrian city of Deraa as a government crackdown on dissent continues.
It said a humanitarian mission had not been allowed access to the city, and a UN agency had been unable to get medical supplies to refugees.
Deraa has been cut off for the past two weeks, after troops and tanks were sent in to restore government control.
Meanwhile, the EU has announced an arms embargo on Syria.
In a statement, the bloc said it was banning the shipment to Syria of "arms and equipment that could be used for internal repression".
Thirteen "officials and associates of the Syrian regime" are banned from travelling anywhere in the 27-nation union and have had their assets in EU countries frozen.
The UN announced last week that President Bashar al-Assad had agreed to a request from the global body's Secretary-General, Ban Ki-moon, for a humanitarian mission to be sent to Deraa, where scores of protesters are reported to have been killed in recent weeks.
But UN humanitarian chief Valerie Amos told reporters on Monday: "We were expecting to go in with a mission to Deraa yesterday. That was postponed by the government."
She said no reason had been given, but added that she had been assured relief teams would be allowed to "go in later this week, which I very much hope will be possible".
At the same time, the UN agency that looks after Palestinian refugees, Unrwa, said that it too had been unable to get emergency medical supplies to the 30,000 or so refugees in the Deraa area.
It said it was particularly concerned for 120 patients there who depend on the agency for supplies of insulin.
Unrwa spokesman Christopher Gunness said the agency had expressed its concern to the Syrian government, and hoped to be able to resume operations soon.
The Syrian authorities announced last week that troops were being pulled out of Deraa, where dozens of people have been reported killed and many hundreds arrested.
But the city remains surrounded and cut off, and military operations are continuing in the surrounding countryside.
Earlier, there were reports of violence in other areas of Syria:
- Heavy gunfire was heard in a western suburb of the Syrian capital, Damascus, after the army cordoned off the area, human rights activists said
- Clouds of black smoke could also be seen over Muadhamiya, and an activist told the BBC that three people had been killed there
- More security forces were reported to have moved into the central city of Homs, where troops backed by tanks have been raiding houses and arresting people since Saturday night
- Security forces are also said to be continuing their efforts to crush anti-government protests in the coastal town of Baniyas
Foreign journalists have not been allowed to enter Syria, so reports from the country are difficult to verify independently.
Correspondents say a key tactic of the authorities has been to isolate and intimidate people in areas where anti-government resistance is strongest.
The army says operations against "armed terrorists" are continuing.
Six soldiers and police had been killed and others wounded on Sunday in Homs, Baniyas and villages around the southern city of Deraa, it added.
And Syrian state TV showed pictures of a minibus in which 10 Syrian workers coming home from Lebanon were said to have died when it was ambushed near Homs by gunmen early on Sunday.
The UK-based Syrian Observatory for Human Rights says 621 civilians and 120 security personnel have been killed since pro-democracy protests began in March. Another rights group, Sawasiah, says more than 800 civilians have been killed.
The government disputes the civilian toll and says about 100 soldiers have been killed.
The unrest poses the most serious challenge to President Assad since he succeeded his father, Hafez, in 2000.
A mobile phone snapshot, reportedly taken in Qamishli on 29 April, shows protesters carrying banners written in Arabic and Kurdish demanding democracy.
Razan, who is a resident of Damascus, tells the BBC about violence and protests around Syria.
This unverified video seems to show a peaceful protest in Talbisah. Moments into the footage, tanks fire on unarmed civilians. Wyre Davis reports.
Residents of Deraa walk past a burnt-out building. It follows shelling by troops in what human rights groups say was an intensified crackdown on protests in recent weeks.
Syrian army vehicles were photographed near Homs and broadcast on the Syrian opposition internet channel Sham SNN on 11 May.