Syrian rebels battled President Bashar al-Assad's forces on the edge of central Damascus on Wednesday, opposition activists said, seeking to break through to the heart of the capital.

Their offensive aims to break a stalemate in the city of 2 million people, where artillery and airstrikes have prevented rebels entrenched to the east from advancing despite their capture of army fortifications, the activists said.

"We have moved the battle to Jobar," said Captain Islam Alloush of the rebel Islam Brigade. The district links rebel strongholds in the suburbs with the central Abbasid Square.

"The heaviest fighting is taking place in Jobar because it is the key to the heart of Damascus," he said.

Assad, battling to crush a 22-month-old uprising in which 60,000 people have died, has lost control of large parts of the country but his forces, backed by air power, have so far kept rebels on the fringes of the capital.

Despite the setbacks, Assad has remained defiant, telling a visiting senior Iranian official on Sunday that Syria was able to confront, "current threats ... and aggression".

That visit came just after Iranian Foreign Minister Ali Akbar Salehi met Syrian opposition leader Moaz Alkhatib in Moscow. Iran is Syria's strongest backer in the region.

Salehi said on Wednesday the Syrian government may be ready to respond positively to Alkhatib's offer of direct peace talks.

"I think that the Syrian government is ready to negotiate with the opposition," he told Egyptian state news agency MENA during a visit to Cairo. But Alkhatib gave the Syrian government until Sunday to release all women detainees otherwise he would regard his offer of dialogue as rejected by Assad.

Meanwhile, the Syrian army foiled the attempt of armed rebels to wage a wide scale offensive on the capital Damascus, the pro-government Al-Watan Daily said on Thursday.