U.S. Secretary of State John Kerry said on Monday Washington was not engaged in a race to complete talks with Iran on its nuclear program and vowed to defend Washington's regional allies against any threats.

Speaking at a news conference with United Arab Emirates (UAE) Foreign Minister Sheikh Abdullah bin Zayed al-Nahayan in Abu Dhabi, Kerry also praised the Syrian opposition's decision to participate in a proposed peace conference as "a big step forward."

"This is not a race to complete just any agreement," Kerry said, adding: "Through diplomacy we have an absolute responsibility to pursue an agreement."

Marathon talks between six major world powers - the United States, Russia, China, Britain,Germany and France - and Iran on Saturday ended without agreement and the sides arranged to meet again on 20 November.

While saying that an agreement with Iran was expected within months, Kerry sought to reassure Washington's Arab allies and Israel that his country would not abandon them.

"We will stand up for and defend our allies in this region against any kind of external threat, so this is a strong strategic relationship and I look forward to continuing our important dialogue...and to strengthening the relationship," Kerry said.