ISTANBUL: The leader of the main Syrian rebel force on Monday threw his weight behind the formation of an interim government to administer rebel-held areas as heavy fighting broke out in the Syrian capital and several suburbs.

At least three mortar shells struck central Damascus, the seat of President Bashar Assad's power. The pro-government's Al-Ikhbariya TV said one of the shells fell in Muhajireen district near Tishreen Palace, one of three palaces that Assad uses in the capital.

Activists also reported that mortar shells struck near state security agencies in al-Barakmeh district and close to the Higher Education Ministry in Mazzeh district.

At least 26 people died in the fighting in Damascus and its suburbs, according to the Britain-based Syrian Observatory for Human Rights.

Also on Monday, Assad's fighter jets struck targets near the town of Arsal, Lebanon, according to Lebanon's state-run National News Agency. The two countries share a porous border and it was unclear if the shelling occurred inside Lebanon.

The strikes came just days after Damascus warned Beirut to stop militants from crossing the border to fight alongside the rebels.

Lebanon has been on edge since the uprising against Assad began in March 2011. Gunmen on opposing sides of Syrian civil war have frequently clashed in Lebanon, raising concerns of a spillover.

Syria's conflict began with political protests in mid-March, 2011, and has since become a civil war, with hundreds of rebel groups fighting Assad's forces. The U.N. says more than 70,000 people have been killed and millions have been pushed from their homes by the violence.

In Istanbul, Gen. Salim Idris told reporters that fighters affiliated with his Free Syrian Army, the main rebel group, will work under the umbrella of an interim government and protect its members.

"We recognize the coalition as our political umbrella and we hope this government can be formed unanimously and that this government will exercise its powers in all of Syria," said Idris, the chief of staff of the FSA. "We consider it the only legal government in the country."

Idris' comments sought to boost efforts by the opposition's Syrian National Coalition to choose a prime minister who will form an interim government. The coalition has failed to take such a step twice before, but members say there now appears to be more agreement that such a step is necessary.

The coalition's 73 members meeting in Istanbul are expected to elect an interim prime minister from 12 candidates. The vote is expected by Tuesday.

Idris' comments also sought to portray his group as the most powerful and organized rebel formation in Syria. It remains unclear, however, how many of the hundreds of rebel brigades fighting Assad's forces follow Idris' commands or are linked to his group.

Some of the most effective rebel groups are Islamic extremists who have developed their own support networks. One of them, Jabhat al-Nusra, has been designated a terrorist group by the United States and is said to be linked to al-Qaida.

Idris said his group has no relationship with Jabhat al-Nusra.

He renewed his call for world powers to arm rebel fighters, saying his group would insure that the weapons don't fall into the wrong hands - a prime concern of the U.S. and other powers, especially given Syria's southern frontier with Israel.

"We have the power and the organizational capacity to control the movement of these weapons and keep them in safe, trusted hands," he said.

Last week, Britain and France urged the European Union to lift a ban on arming Syrian rebels, but with other EU members opposed, no decision was reached.

The U.S. is reluctant to send weapons, fearing they might end up with extremists, but U.S. Secretary of State John Kerry said Monday that the Obama administration won't hold anyone back from doing so.

"The United States does not stand in the way of other countries that made a decision to provide arms, whether it's France, or Britain or others," Kerry said in Washington.

The longer Syria's violence goes on, the greater the danger of its institutions collapsing and extremists obtaining chemical weapons, Kerry said.

Idris, meanwhile, suggested other types of possible military aid, saying his group would welcome Lebanese or international forces along Syria's border with Lebanon. He also called for rebel units to be trained to seize Syria's chemical weapons.

"The chemical weapons in Syria are not secured," he said, warning that they could be used against rebels or given to "rogue groups, like the Lebanese militant group Hezbollah."

"We ask the international community to help us train special forces that can secure these weapons and keep them from falling into the hands of extremists," he said.