Obama vows 'eternal' defence of Israel

US President Barack Obama on Wednesday pledged an "eternal" alliance with Israel in the face of the Iranian threat, saying he accepted the Jewish state would not defer to Washington on the question of how to handle it.

At the start of a historic trip aimed at easing past tensions over Iran, Obama reached out with a message of reassurance about his commitment to Israel's security in a bit to offset scepticism over his strategy for confronting Iran.

Both he and Israeli Prime Minister Benjamin Netanyahu appeared to be making a visible effort to turn a new page in their personal relationship, which has been marred by several public spats.

"The United States is proud to stand with you as your strongest ally and your greatest friend," Obama said at a lavish welcoming ceremony at Tel Aviv's Ben Gurion airport after Air Force One rolled to a halt to a peal of military trumpets.

"Our alliance is eternal, it is for ever," he said, as hundreds of US and Israeli flags snapped loudly in the wind.

The long-awaited visit, the first foreign tour of Obama's second term, comes just days after the installation of a new rightwing Israeli government which faces key challenges of how to handle Iran's nuclear drive, the growing threat from Syria and peace with the Palestinians.

"It's good to be back in The Land (Israel)," Obama said in Hebrew after being greeted on the red carpet by Netanyahu and President Shimon Peres.

Before leaving the airport, the US leader came face-to-face with Israel's preoccupation with security, visiting a mobile battery of the US-funded Iron Dome missile defence system.

He then headed to Peres's residence and met by a troupe of flag-waving children, five of whom serenaded him with a rendition of the hit-musical number "Tomorrow" in Hebrew, English and Arabic.

After a quick tour of Peres's garden, the two got down to business with an hour-long meeting which Obama later said had focused on Iran, the peace process and the turmoil in the Middle East.

Obama then went into several hours of talks with Netanyahu, later telling a news conference he did not expect Israel to defer to Washington on the question of how to handle Iran.

Asked if he had pressed the Israeli leader to hold off on threats to attack Iran's nuclear facilities, he said: "I would not expect the prime minister to make a decision about his country's security and defer that to any other country."

He said there was no greater decision for a leader than to give the "awesome" decision to order military action, but said he did not know what Israel was planning to do.

It was not immediately clear whether the two men had narrowed their differences over when Iran was likely to cross the point of no return and acquire the ability to build a weapons capability.

Obama has said Iran will not be able to build a nuclear weapon for "over a year or so" but Netanyahu believes it could have the capacity to produce a bomb within months.

Peres said he was confident that Obama would make good on his pledge to prevent a nuclear Iran.

"We trust your policy which calls first to try by non-military means with a clear statement that there are other options on the table," he said, expressing a confidence rarely voiced by Israeli officials.

Obama also issued a stark warning to Syria about using chemical weapons against its civilians, saying it would be a "grave and tragic mistake" and a "game-changer."

"The Assad regime must understand they will be held accountable," he said in remarks a day after the regime of Bashar al-Assad traded accusations with rebel forces over the use of chemical agents in an attack in the northern province of Aleppo which killed 31.

So far, Washington says it has seen no evidence that such weapons were used, but Obama said he was "deeply sceptical" of any claim that opposition forces were involved.

Peres also warned about chemical arms falling into the wrong hands. "We cannot allow those weapons to fall into terrorists' hands -- it could lead to an epic tragedy," he said.

Obama's arrival comes after a two-and-a-half year deadlock in peace talks between Israel and the Palestinians, although Netanyahu insisted that his newly-inaugurated government remained committed to the two-state solution.

"Israel remains committed to the solution of two states for two peoples."

Obama is expected to hold talks in Ramallah with Palestinian president Mahmud Abbas at 0900 GMT on Thursday, although he has made clear that he is here "to listen" to both sides rather than launch any new peace initiative.

The Palestinians are hoping he will help broker the release of more than 1,000 prisoners held by Israel and also free up $700 million in blocked US aid.