By Massimiliano Di Giorgio and Steve Scherer
ROME (Reuters) - President Sergio Mattarella won reassurances from Italy's main political leaders on Thursday that they fully supported NATO after the crisis in Syria exposed rifts in the country's traditional pro-Western stance.
While rallying behind NATO, parties of all colours said Italy should not take part in any military action against Syria following accusations that President Bashar al-Assad's forces had launched a poison gas attack on a rebel-held town last week.
Mattarella is seeking to put together a coalition government following inconclusive elections in March, but his task has been complicated by U.S. threats to strike out at Assad, who is backed by Russia and Iran.
Although Italy has traditionally been one of Washington's most loyal allies, two parties that performed strongly at the March ballot -- the far-right League and anti-establishment 5-Star Movement -- have called for much warmer ties with Russia.
League leader Matteo Salvini surprised Mattarella's office on Wednesday by denouncing reports of the Syrian chemical attack as "fake news" and criticising U.S. President Donald Trump for promising to punish Syria's leadership.
"There's no doubt (Mattarella) is worried," a source in the Italian president's office told Reuters.
Clearly on the orders of the head of state, party leaders all voiced their support for Italy's Western partners as they trooped out of government consultations on Thursday.
"I reaffirm ... that our intention is to remain close to our allies," 5-Star leader Luigi Di Maio told reporters, adding: "It is also clear that the end of the fighting in Syria will not be brought about by a military victory but by diplomacy."
Salvini said Italy had an obligation to remain "loyal" to NATO, but reiterated: "We are firmly opposed to any unilateral actions" in Syria.
Italy is a member of the U.S.-led alliance and hosts important NATO and U.S. military bases.
Italy's caretaker administration, led by the centre-left Democratic Party (PD), which was roundly defeated last month, has condemned the Syrian gas attack, but has ruled out any direct Italian involvement in mooted military retaliation.
However, in an apparent reference to the use of its bases, Prime Minister Paolo Gentiloni said in a statement on Thursday: "Italy will continue to offer logistical support to allied forces."
But even that could cause political problems in Italy.
The centre-right Forza Italia party, which is allied to the League and led by former prime minister Silvio Berlusconi, said use of the bases for action in Syria should be allowed only if sanctioned by the U.N. Security Council.
Russia has consistently vetoed any criticism of Assad in the Security Council and this week rejected a U.S.-drafted resolution that would have created a new inquiry to ascertain blame for chemical weapons attacks in Syria.
(Editing by Crispian Balmer and Richard Balmforth)