Syrian President Bashar Assad has denied responsibility for a chemical weapons attack on Syrian civilians and said evidence was not conclusive that there had been such an attack.

"There has been no evidence that I used chemical weapons against my own people," US TV station CBS reported Mr Assad as saying during an interview conducted in Damascus. The full interview is due to be shown on American television today.

Mr Assad spoke as the Obama administration was pressing its case in the United States for congressional authorisation of a US strike against Syria in response to the sarin gas attack in August that Washington said killed more than 1400 Syrians, including several hundred children.

The Obama administration has accused Mr Assad's forces of carrying out the attack but Mr Assad has blamed the rebels.

Passage by Congress of the authorisation for use of force is by no means certain. Votes are expected this week.

Mr Assad would neither confirm nor deny that Syria has chemical weapons and called on the United States to produce evidence of his involvement.

He warned that if there was a military strike by the US it could degrade his military and tip the balance of the conflict in favour of the rebels. He also said any attack would be met with retaliation by those aligned with Syria.

The new foreign minister of Syria's close regional ally Iran declared that any use of force without UN authorisation would be illegal.

During a visit to Iraq, Mohammad Javad Zarif said: "I do not know why those who say all options are on the table do not understand the fact that civilised countries 65 years ago ... rejected in the charter of the United Nations [the] resort to force as an illegal practice."

Meanwhile, a senior MP has caled for a watchdog to examine why a chemical that can be used to make the deadly sarin nerve agent was sold by UK firms to Syria with Government permission.

Supplies of sodium fluoride were exported between 2004 and 2010, during the build-up to the brutal civil war, when Assad was believed to be stockpiling chemical weapons.

The substance is an ingredient of sarin, which UK experts have established was used in the attack.

Tory Richard Ottaway, who chairs the Foreign Affairs Select Committee, said: "Any sensible person would be concerned if an ingredient of sarin gas was exported from the UK to Syria.

"I think this is something for the committee."

The Department for Business, Innovation and Skills (Bis) said there was no evidence the chemicals, which it licensed for export for use in cosmetics and healthcare, were diverted to weapons programmes.

"These licence applications, which predate the current conflict and EU sanctions, were rigorously assessed and determined to be for legitimate commercial use," a spokesman said. The quantities involved were "commensurate with the stated end use", it said.

Bis granted a number of licences to specialist firms allowing them to deliver sodium fluoride to Syria before the current conflict and European Union (EU) sanctions. Licences were issued in July 2004, September 2005, March 2007, February 2009 and May 2010, the year before the civil conflict erupted.

Last week, UK scientists at Porton Down said they found positive evidence of sarin on samples of soil and clothing reportedly retrieved from the site of the attack. Professor Alastair Hay, a toxicology expert at Leeds University, told the Mail on Sunday: "The Government's approval of sodium fluoride sales to Syria when it was widely suspected the regime was stockpiling dangerous substances is deeply disturbing."