* State TV reports casualties, but says Halki unhurt
* Attack occurred in capital's shrinking "security square"
* Rebels, army in conflict that has killed 70,000
* U.N. urges full access to Syria for chemical arms team (Adds Obama, Putin, U.N. reaction)
By Dominic Evans
BEIRUT, April 29 Syria's prime minister survived a bomb attack on his convoy in Damascus on Monday, as rebels struck in the heart of President Bashar al-Assad's capital.
Six people were killed in the blast, the British-based Syrian Observatory for Human Rights said. Previous rebel attacks on government targets included a December bombing which wounded Assad's interior minister.
U.N. Secretary-General Ban Ki-moon condemned the bombing, which he described as a "terrorist attack."
As prime minister, Wael al-Halki wields little power but the attack highlighted the rebels' growing ability to target symbols of Assad's authority in a civil war that, according to the U.N., has cost more than 70,000 lives.
Assad picked Halki in August to replace Riyadh Hijab, who defected and escaped to neighbouring Jordan just weeks after a bombing killed four of the president's top security advisers.
Monday's blast shook the Mezze district soon after 9 a.m. (0600 GMT), sending thick black smoke into the sky. The Observatory said one man accompanying Halki was killed as well as five passers-by.
State television showed firemen hosing down the charred and mangled remains of a car. Close by was a large white bus, its windows blown out and its seats gutted by fire. Glass and debris were scattered across several lanes of a main road.
"Dr. Wael al-Halki is well and not hurt at all," state television said.
It later broadcast footage of Halki, who appeared composed and unruffled, chairing what it said was an economic committee.
In comments released by the state news agency SANA but not shown on television, Halki was quoted as condemning the attack as a sign of "bankruptcy and failure of the terrorist groups", a reference to the rebels battling to overthrow Assad.
Mezze is part of a shrinking "Square of Security" in central Damascus, where many government and military institutions are based and where senior officials live.
Sheltered for nearly two years from the destruction ravaging much of the rest of Syria, it has been sucked into violence as rebel forces based to the east of the capital launch mortar attacks and carry out bombings in the centre.
Assad has lost control of large areas of northern and eastern Syria, faces a growing challenge in the southern province of Deraa, and is battling rebels in many cities.
But his forces have been waging powerful ground offensives, backed by artillery and air strikes, against rebel-held territory around the capital and near the central city of Homs which links Damascus to the heartland of Assad's minority Alawite sect in the mountains overlooking the Mediterranean.
As part of that counter-offensive, Assad's forces probably used chemical weapons, the United States and Britain have said.
However, the trans-Atlantic allies, whose 2003 invasion of Iraq to overthrow Saddam Hussein was based in part on flawed intelligence about an Iraqi programme of weapons of mass destruction, have been cautious in their accusations.
Ban said on Monday that investigators have been gathering and analysing available information on alleged chemical attacks in Syria, but full access to the war-torn country is essential for a "credible and comprehensive inquiry."
Assad's government has refused to give the U.N. inspection team the kind of unfettered access inside Syria that Ban is demanding. As a result, the team has yet to deploy to Syria.
A Western diplomat said British officials had shown the head of the U.N. inspection team, Ake Sellstrom of Sweden, evidence on which London based its assertion that there was "limited but growing" evidence of chemical weapons use - possibly the nerve agent sarin - by Syrian troops.
But Sellstrom found the evidence inconclusive, said the diplomat, speaking on condition of anonymity.
President Barack Obama repeated U.S. concerns about Syrian chemical weapons in a phone conversation with Russian President Vladimir Putin on Monday, the White House said, adding that the two leaders agreed to stay in contact.
The United States is trying to determine the facts around alleged Syrian use of chemical weapons. Last week U.S. officials said they had "varying degrees of confidence" that such weapons were used in Syria, which if proven with certainty could trigger unspecified U.S. action against the Syrian government.
Despite congressional pressure to do more to help the rebels, the U.S. president has made clear he is in no rush to intervene on the basis of preliminary evidence.
A U.N. team of experts has been waiting to travel to Syria to gather field evidence, but has yet to win agreement from Syrian authorities who want it to investigate only government accusations of chemical weapon use by rebels in Aleppo province.
Russia, which has criticised Western and Gulf Arab support for the anti-Assad fighters, said that attempts by Western countries to expand the U.N. inquiry to cover rebel accusations of Syrian government use of chemicals in Homs and Damascus mounted to a pretext to intervene in the civil war.
The U.N. said in February that around 70,000 people had been killed in Syria's conflict. Since then activists have reported daily death tolls of between 100 and 200.
Five million people have fled their homes, including 1.4 million refugees in nearby countries, and financial losses are estimated at many tens of billions of dollars.
The Beirut-based U.N. Economic and Social Commission for Western Asia estimates that 400,000 houses have been completely destroyed, 300,000 partially destroyed and a further half million have suffered some kind of structural damage. (Additional reporting by Thomas Grove in Moscow and Michelle Nichols and Louis Charbonneau in New York; Editing by Alistair Lyon, Robin Pomeroy and Paul Simao)