The Syrian government on Tuesday maintained a block on giving UN experts unconditional access to investigate claims that chemical weapons have been used in the country's civil war.

Amid mounting pressure over possible use of the banned arms, Syria's UN envoy insisted any inquiry be limited to claims that opposition rebels fired chemical weapon shells near Aleppo last month. He said a "chemical material" was used in a new incident on Monday.

UN leader Ban Ki-moon is demanding that international experts also investigate the alleged use of the weapons at Homs in December.

The showdown became more tense as US President Barack Obama said Tuesday that proof of the use of chemical arms would trigger a "rethink" of his reluctance to order a military intervention in the two-year-old conflict.

The United States said last week it believed the Syrian government has used chemical weapons but was awaiting definitive proof.

The UN leadership has sent at least four letters to President Bashar al-Assad's government demanding "unfettered" access to investigate all claims.

"To date we do not yet have what we need -- mutually speaking -- between the Syrian authorities and the (UN) office for disarmament affairs for that mission to be able to go into Syria," said UN spokesman Martin Nesirky.

The UN team, led by Swedish scientist Ake Sellstrom, is already studying evidence brought out of Syria, according to diplomats.

Syria first asked for the inquiry last month after accusing opposition rebels of using chemical weapons at Khan al-Assal near Aleppo on March 23 in an attack in which authorities say more than 30 people died.

Britain and France, however, demanded that any UN inquiry also look into the Syrian opposition's claim that the government used chemical arms at Homs on December 23.

Nesirky welcomed the government's assurances on access to Khan al-Assal, but added: "Such cooperation should also be extended to Homs, the site of the other allegation."

He told reporters the Syrian government had sent a new letter on Monday "which reiterated their position regarding the investigation of the incident at Khan al-Assal only."

Syria's UN envoy Bashar Jaafari said the Assad government had been a victim of "trumped up charges and fraudulent accusations" over the use of chemical weapons.

He said the Damascus authorities should be given access to information that Britain and France have given to the United Nations to back their calls for an inquiry.

"There cannot be, at any time, a mission of investigation moving freely on Syrian territory just on the basis of letters containing allegations," Jaafari told a news conference.

He said an investigation of Khan al-Assal could be started in less than 24 hours. "The Syrian government does not close the door to looking at any other allegations" but must see the details first.

Jaafari said "terrorist groups" had got into the town of Salaqeb near Idlib on Monday and "spread seemingly the contents of plastic bags containing a kind of powder which must be most probably a chemical material."

The ambassador said many people were affected by the "heinous and irresponsible act" that was an attempt to "implicate the Syrian government on a false basis."

There were no independent reports of the attack or whether a chemical was used.