Gunmen have killed an Iranian commander in Syria as rebels shot down two fighter jets and overran a town, dealing further setbacks for forces loyal to President Bashar al-Assad.

The ambush on Thursday that killed the Revolutionary Guards commander, the downing of the aircraft in the northwest and the seizure of the town of Shadadeh near the Iraqi border amounted to four straight days of battlefield successes for the rebellion.

The insurgents overran a military air base in Aleppo province on Tuesday, after taking control of Syria's largest dam in the neighbouring province of Raqa the day before.

The latest setbacks came after US Secretary of State John Kerry said Assad needed to abandon hopes of riding out the war and accept the "inevitability" of his departure.

On Thursday, Iran's Revolutionary Guards said one of its commanders, Hassan Shateri, was "martyred... at the hands of Zionist regime mercenaries and backers" while travelling by road between Damascus and Beirut.

The Guards said he had also headed the Iranian Committee for the Reconstruction of Lebanon, and the Iranian embassy in Beirut gave a similar account.

Iranian Foreign Minister Ali Akbar Salehi strongly condemned the killing as a "terrorist act" and paid tribute to "this commander of Islam and his tireless efforts in reconstruction."

A strong ally of the Damascus regime, Tehran often refers to rebels fighting Assad's troops as "terrorists" with ties to arch-foe Israel.

Syria's rebellion flared after Assad's forces launched a bloody crackdown on peaceful democracy protests that erupted in March 2011, inspired by the Arab Spring uprisings, but it has become increasingly dominated by Islamist groups.

In London, British Foreign Secretary William Hague warned that Islamist militants from Western countries who had gone to fight in Syria could launch terrorist attacks when they return.

Hague said Syria had become the "number one destination" for jihadists worldwide.

One of the most prominent radical Islamist groups, the Al-Nusra Front, took the town of Shadadeh in the oil-rich northeastern province of Hasakeh on Thursday.

"After three days of fierce battles against the army, Al-Nusra Front fighters have seized control of Shadadeh," said the Syrian Observatory for Human Rights.

Fighting and car bombings by the jihadists killed more than 100 troops in three days, during which 30 Al-Nusra Front fighters also died, five of whom were from Kuwait, said the Observatory.

Elsewhere, the army made its own advances, taking a district in the central city of Homs after weeks of heavy clashes.

The victory comes a week after the army took control of Kafraya on the southwestern outskirts of Homs, a city opposition activists refer as "the capital of the revolution."

In Washington, Kerry said the death toll from the conflict may have reached 90,000, citing figures provided by his Saudi counterpart Prince Saud al-Faisal -- sharply up on recent UN estimates of nearly 70,000 people killed.

Kerry had earlier suggested taking renewed steps to urge Syrian ally Russia to bring more pressure on Assad to quit, after Moscow insisted it was ready to host talks with both sides.

Syria welcomed Russia's invitation, while stressing its foreign minister would not meet Ahmed Moaz al-Khatib, the opposition chief who has offered to hold peace talks with regime officials without blood on their hands.

Meanwhile the United Nations denied reports that new peace plan is being drawn up for Syria involving the creation of a senate to oversee a power transition.

Saudi newspaper Asharq al-Awsat reported that it had obtained a copy of a plan aimed at ending Syria's 23-month conflict, saying that it had been drafted "under UN supervision."

Members of Syria-based opposition groups tolerated by the regime of President Bashar al-Assad helped draft the plan, the paper added.

But the United Nations said in a statement that "neither the secretary-general nor the joint special representative (international envoy Lakhdar Brahimi) have any knowledge of the so-called plan."

"However, Mr Brahimi and his team continue to work with all stakeholders toward a peaceful settlement of the Syrian conflict."

The world body also noted that Ban and Brahimi support opposition chief Ahmed Moaz al-Khatib's offer to hold peace talks with regime officials without blood on their hands.