Syria's opposition coalition met in Istanbul to choose its first prime minister on Monday, as Syrian warplanes bombed northern Lebanon for the first time in what Washington labelled "a significant escalation" of the conflict.

A former Syrian agriculture minister, an economist and a communications executive lead the race to head a rebel government ahead of a vote by the umbrella Syrian National Coalition that could change the course of Syria's civil war.

Back at home, Damascus warplanes were accused of bombing the border area between Syria and Lebanon, reportedly targeting the town of Arsal in east Lebanon, where many residents back the rebels' uprising.

A high-ranking Lebanese army official told AFP four missiles had been fired, apparently aiming for Syrian rebels positions inside Lebanon.

A top US official confirmed the strikes, denouncing them as "unacceptable".

"We can confirm... that regime jets and helicopters did fire rockets into northern Lebanon, impacting Wadi al-Khayl, near the border town of Arsal," State Department spokeswoman Victoria Nuland told reporters.

"This constitutes a significant escalation in the violations of Lebanese sovereignty that the Syrian regime has been guilty of. These kinds of violations of sovereignty are absolutely unacceptable."

She reminded Damascus that UN resolutions called for the strict respect of the sovereignty and territory of Lebanon.

The Syrian conflict is now in its third year, with about 70,000 people killed and millions forced from their homes, according to the United Nations.

In Istanbul, if the opposition meeting leads to the election of a premier as is expected, his first task would be to appoint an interim government, which would be based inside parts of Syria freed from the control of Damascus.

"The vote is likely to take place today. We are still discussing differences are over how capable and loyal to the revolution each candidate is," Coalition deputy head Soheir Atassi told AFP.

While it would boost the opposition's international credibility and its reputation among dissidents on the ground, a rebel government would reduce chances of talks with the regime of President Bashar al-Assad, which Coalition leader Moaz al-Khatib had proposed in January.

"The regime refused that negotiation process, so that is something off the table at this point," said Coalition spokesman Khaled al-Saleh.

Pro-Assad daily Al-Watan was quick to slam the Coalition's bid to form a government, branding it "delirious and confused".

The 73-member Coalition is expected to hold an initial vote, followed by a run-off between the top two candidates. The winner will then choose a cabinet, which must be approved by the Coalition.

Free Syrian Army chief Selim Idriss said they would support and "work under the umbrella of this government", reducing concerns that the rebels could from the outset be opposed to an interim civilian authority.

Idriss also repeated calls to the West to arm the insurgents, adding he could guarantee that "these weapons will not fall into the wrong hands".

European Union leaders are due to meet this week to discuss easing an EU arms embargo, amid statements from Paris and London that it is time to start arming the Syrian opposition.

The United States said Monday it would not block European moves to arm Syrian rebels battling Assad.

"President (Barack) Obama has made it clear that the United States does not stand in the way of other countries that have made a decision to provide arms, whether it's France or Britain or others," US Secretary of State John Kerry told reporters.

On the ground in Syria, regime forces resumed shelling parts of Damascus as security forces carried out arrests of students in housing attached to Damascus University, the Syrian Observatory for Human Rights said.

Warplanes also carried out raids across the country, including in the Baba Amr neighbourhood of central Homs. Nationwide, at least 71 people were killed Monday, the group said.

-- No 'Skype government' --

The United States is believed to oppose the creation of an interim government, fearing it could hamper efforts to start a dialogue with the regime, but the process has been backed by Turkey and much of the Arab League, which may influence the choice.

Out of the 12 candidates, opposition sources said former agriculture minister Asaad Mustafa, economist Osama Kadi, and communications executive Ghassan Hitto were frontrunners for the vote.

The Coalition agrees that the premier and his government would have to be based inside Syria, spokesman Saleh told reporters. "A Skype government is not going to work."

Aleppo-born Kadi, founder of the Syrian Centre for Political and Strategic Studies in Washington, is favoured for his technocratic background, as is Hitto, who has lived in the US for decades.

Mustafa brings eight years of experience as a minister under Syria's late president Hafez al-Assad.