Syrian security forces are on high alert after an attack in the diplomatic district of the capital Damascus.
The building hit by the blast was no longer occupied by the UN
Police are controlling access to the Mazzeh area, where a bomb exploded and four people died as security forces battled gunmen on Tuesday night.
A policeman, a passer-by and two alleged bombers died in the incident.
Local media have shown pictures of a weapons cache discovered after the unusual clash which shocked Syria - a tightly policed one-party state.
The Syrian government said the violence was a symptom of the unrest afflicting the entire region.
"It seems it is difficult to maintain 100% security in a tense region which has turned into an inferno," said the state-run newspaper Al-Baath, referring to the Israeli-Palestinian conflict and violence in neighbouring Iraq.
The BBC's Kim Ghattas in Damascus says political violence was effectively banished from the country after an Islamist uprising was quashed in the early 1980s.
Our correspondent says Syrian media were unusually swift in reporting the incident but it is still not clear what the militants' intended target was.
A bomb exploded and police clashed with gunmen in the west of the city, close to the Iranian and Canadian embassies
Security forces reportedly clashed with militants who opened fire indiscriminately after planting a bomb under a parked car.
A building formerly used by the UN was burnt out by the blast.
"A clash ensued between the security forces and the terrorist group, which fled aboard a second car throwing grenades at the security forces," an interior ministry official told the state news agency.
A policeman and a woman passer-by died along with two bombers. Two others were wounded.
Canadian and British officials said their embassies were undamaged and they were unaware of any casualties among their citizens.
The BBC's Middle East analyst Roger Hardy says there are three possible culprits for Tuesday night's attack.
They might have been members of a resurgent Muslim Brotherhood, the pan-Arab outfit that spearheaded the Islamist revolt two decades ago.
They might be part of Syria's Kurdish minority, which was involved in street clashes in March.
Or finally, they could be agents of al-Qaeda - though this seems doubtful given the Syrian government's apparent disdain for US policies in the region.