President Barack Obama has vowed to back Gulf allies against any "external attack," seeking to reassure them of Washington's commitment to their security amid Arab anxiety over US-led efforts to reach a nuclear deal with Iran.

Obama, hosting the six-nation Gulf Cooperation Council for a rare summit at the Camp David presidential retreat, pledged that the US would cooperate with them to address what he called Iran's "destabilising activities in the region".

"The United States will stand by our GCC partners against external attack and will deepen and extend cooperation that we have," Obama told reporters, with Gulf leaders standing by his side at the end of the talks.

Obama promised a "concrete series of steps" from the one-day summit as he sought to allay Gulf Arab fears that the potential lifting of international sanctions on Tehran would embolden it in the region and raise the risk of more sectarian strife.

Al Jazeera's Patty Culhane, reporting from Camp David, said Saudi Arabia's foreign minister had said details of the Iran agreement were still being implemented so Riyadh would wait and see what happens.

The leaders issued a joint statement saying that in the event of aggression, the US stood ready to work with the Arab nations "to determine urgently what action may be appropriate, using the means at our collective disposal, including the potential use of military force, for the defence of our GCC partners".

While the US has long provided military support to partners in the Gulf, the joint statement pledged new cooperation on counterterrorism, maritime security, cybersecurity and ballistic missile defence, among other things.

But US officials said the increased US commitments stop short of a formal defence treaty that some of the Gulf countries had sought.

The leaders also agreed to press all parties in Libya to reach political agreement and a national unity government before Ramadan.

Gulf Arab frustration

Differences over US policy toward Tehran, Syria's civil war and the Arab Spring uprisings loomed over Thursday's meetings, which were already clouded by the absence of most of the Gulf's ruling monarchs, who instead sent lower-level officials.

Saudi Arabia's King Salman bin Abdulaziz al-Saud pulled out early, sending Crown Prince Mohammed bin Nayef and Deputy Crown Prince Mohammed bin Salman in his place.

The decision was widely interpreted as a snub that reflected the GCC's frustration with the Obama administration.

The White House has said such decisions were not intended as slights and has portrayed the summit as more than just a symbolic event.

In an interview to Al Jazeera's Patty Culhane on Wednesday, Ben Rhodes, US deputy national security adviser for strategic communication, said the US was committed to the defence of the GCC countries but a formal treaty would not happen in the near future.

"A treaty is not what we're looking for. It took decades to build NATO and the Asian allies but we can provide clear assurances that we will come to their defence," he said, alluding to a prospective alliance with the GCC members.

Sunni Arab leaders are concerned that lifting Western sanctions as part of a nuclear deal with Iran would empower Tehran to act in further destabilising the region, especially in volatile countries such as Syria, Yemen and Iraq.

The Obama administration would like GCC support, or at least a toning-down of any criticism, for the deal to help convince a sceptical US Congress it has broad backing in the region.

Adding weight to Arab concerns, Iran's Revolutionary Guard Corps navy fired warning shots over a Singapore-flagged cargo vessel in international waters in the Gulf on Thursday.

The shots prompted the vessel to flee into the UAE's territorial waters, according to US officials.

Rhodes said the incident highlighted "exactly" why Gulf Arab states were concerned about Iran's behaviour.

