Syrian President Bashar Assad has signed a law ratifying a $1 billion credit line from top regional ally Iran, Syria's state news agency SANA said in a newsflash on Wednesday, funds which will help ease economic strains from the costly war.

The agreement was between two state-owned banks, the Syrian Commercial Bank and Export Development Bank of Iran, it said. Iran signed a previous $3.6 billion credit line with Iran in July 2013 which has been used up mostly for oil imports, bankers have said.

The new deal was signed on May 19 and approved by the Syrian parliament on Tuesday, SANA said. The money would be used for funding imports of goods and commodities and implementing projects, it said, without giving details.

Tehran's financial aid has been seen as pivotal to the Syrian government and the economy, which has more than halved in the four years since the conflict erupted, researchers say.

Iran is believed to have sent dozens of military advisers and President Hassan Rouhani has said Tehran will back Assad "until the end of the road". Several senior Iranian military figures have been killed in recent months.

Syrian officials and businessmen said in May last year that Damascus had been negotiating a new credit facility with Iran as the old credit line was being used up.

Syria's economy has been hurt by depletion of foreign reserves that were estimated at around $16-18 billion before the crisis. The country had been earning some $2.5 billion a year from oil exports before the war.