US Defence Secretary Leon Panetta. - File Photo.

WASHINGTON: The Obama administration said on Friday it is warning Iran through public and private channels against any action that threatens the flow of oil from the Persian Gulf.

Spokesmen were vague on what the United States would do about Iran's threat to block the strategic Strait of Hormuz, but military officials have been clear that the US is readying for a possible naval clash.

That prospect is the latest flashpoint with Iran, and one of the most serious.

Although it currently overshadows the threat of war over Iran's disputed nuclear program, perhaps beginning with an Israeli military strike on Iran's nuclear structure, both simmering crises raise the possibility of a shooting war this year.

''We have to make sure we are ready for any situation and have all options on the table,'' Defense Secretary Leon Panetta said, addressing a soldier's question Thursday about the overall risk of war with Iran.

Navy officials said that in separate incidents January 6, three Iranian speedboats each armed with a mounted gun briefly chased after a US Navy ship just outside the Gulf near the Strait of Hormuz and a US Coast Guard cutter in the northern Gulf. No shots were fired and the speedboats backed off.

For several reasons, the risk of open conflict with Tehran appears higher in this election year than at any point since President Barack Obama took office with a pledge to try to bridge 30 years of enmity.

A clash would represent a failure of US policy on several fronts and vault now-dormant national security concerns into the presidential election contest.

The US still hopes that international pressure will persuade Iran to back down on its disputed nuclear program, but the Islamic regime shows no sign it would willingly give up a project that has become a point of national pride.

A bomb, or the ability to quickly make one, could also be worth much more to Iran as a bargaining chip down the road.

Time is short, with Iran making several leaps toward the ability to manufacture a weapon if it should choose to do so.

Iran claims its nuclear development is intended for the peaceful production of nuclear energy.

Meanwhile, several longstanding assumptions about US influence and the value of a targeted strike to stymie Iran's progress toward a nuclear weapon have changed.

For one, the White House is no longer confident it could prevail on Israel not to launch such a strike.

An escalating covert campaign of sabotage and targeted assassinations highlighted by this week's killing of an Iranian nuclear scientist may not be enough to head off a larger shooting war, and could prod Iran to strike first.

The brazen killing of a young scientist by motorcycle-riding bombers is almost surely the work of Israel, according to US and other officials speaking on condition of anonymity to discuss intelligence matters.

The killing on a Tehran street followed the deaths of several other Iranians involved in the nuclear program, a mysterious explosion at an Iranian nuclear site that may have been sabotage and the apparent targeting of the program with an efficient computer virus.

Iranian officials accuse both Israel and the US of carrying out the assassination as part of a secret operation to stop Iran's nuclear program.

The killing came a day after Israeli military chief Lt. Gen. Benny Gantz was quoted as telling a parliamentary panel that 2012 would be a ''critical year'' for Iran, in part because of ''things that happen to it unnaturally.''

Secretary of State Hillary Rodham Clinton and Panetta made a point of publicly denying any US involvement, but the administration tied itself in knots this week over how far to go in condemning an action that could further the US goal of stalling Iranian nuclear progress.

The US position remains that a military strike on Iran's known nuclear facilities is undesirable because it would have unintended consequences and would probably only stall, not end, the Iranian nuclear drive.

That has been the consensus view among military leaders and policymakers for roughly five years, spanning a Republican and Democratic administration.

But during that time Iran has gotten ever closer to a potential bomb, Israel has gotten more brazen in its threats to stop an Iranian bomb by nearly any means, and the US administration's influence over Israel has declined.

Israel considers Iran its mortal enemy, and takes seriously the Iranian threat to wipe the Jewish state from the map.

The United States is Israel's strongest ally and international defender, but the allies differ over how imminent the Iranian threat has become, and how to stop it.

The strained relationship between Obama and Israeli Prime Minister Benjamin Netanyahu plays a role, as does the rise in influence of conservative political parties in Israel.