Yemen's presidential election set for February may be delayed by security concerns, its foreign minister said, raising the prospect that a U.S. and U.N.-backed plan to end months of unrest by easing the president from office may collapse.

The comments - the first suggestion the vote might be postponed - came after Islamist fighters seized an entire city, underscoring U.S. and Saudi fears that chaos born of political crisis may empower al Qaeda in Yemen, which sits alongside key oil and cargo shipping lanes in the Red Sea.

The vote is central to the plan crafted by the Gulf Cooperation Council (GCC), a bloc of Yemen's wealthy neighbours, to ease President Ali Abdullah Saleh from power after nearly a year of protests against his 33-year rule.

Unfortunately, there are a couple of events relating to security, and if they are not solved ... it will be difficult to run the elections on February 21, Foreign Minister Abubakr al-Qirbi, a member of Saleh's General People's Congress (GPC) party, said in an interview shown on Al Arabiya TV on Tuesday.

The opposition coalition that shares power with the GPC in a government tasked with leading Yemen to a vote and ending fighting between Saleh's forces and those of a rebel general and tribal magnates swiftly rejected any delay.

The statement makes clear the practices of President Saleh's regime, which aim to create chaos, said Ghalib al-Odainy, a spokesman for the Joint Meeting Parties (JMP).

He echoed charges that Saleh - long backed by Washington as part of its counterterrorism strategy - was ceding territory to Islamists in order to demonstrate that the end of his rule means anarchy in which al Qaeda will flourish.

These statements make it clear that the handover of Radda was with the complete approval of Saleh's regime, he said, referring to a town about 170 km (105 miles) southeast of the capital Sanaa which Islamist fighters took on Sunday.

The goal is to put the country in chaos and then avoid the Gulf initiative and the presidential elections.

Saleh's camp denies the charges and accuses an Islamist party - once its partner in government - that is powerful in the JMP of being a front for al Qaeda.

Speaking at a news conference in the Ivory Coast capital Abidjan, U.S. Secretary of State Hillary Clinton expressed concern over al Qaeda's activities in Yemen.

We remain focused on the threat posed by al Qaeda in Yemen and will continue to work with our partners there and elsewhere to ensure that AQ does not gain a foothold in the Arabian peninsula through actions that would undermine the stability of Yemen and the region.

ISLAMIST THREAT

Yemen's anti-Saleh protests were largely inspired by the 'Arab Spring' demonstrations that toppled the leaders of Tunisia, Egypt and Libya last year.

The youth activists who have led demonstrations aimed at ousting Saleh reject the transition deal, which would grant him and close aides immunity from prosecution. They want him tried for killing protesters during the uprising.

The interim government - which is to oversee the separation of pro-Saleh forces from rebel army units and tribal militias which have fought one another - has backed a draft immunity law now awaiting parliament's approval.

Saleh earlier this month said he would stay in Yemen, reversing plans to travel to the United States.

We regret that the president has thus far failed to comply with his own commitments to leave the country, to permit elections to go forward that give the people a chance to be heard and be represented, Clinton said.

Forces loyal to General Ali Mohsen - a longtime Saleh confidant who turned on the president as protests against him gained momentum - accused Saleh of handing Radda to Islamists, who also control of much of the southern Abyan province.

The regime is repeating its irresponsible experiment in Abyan to create a security and administrative breakdown in Radda, the rebel units said in a statement. They believe it will ... sabotage implementation of the Gulf initiative.

Any successor to Saleh will face multiple conflicts in the impoverished Arabian Peninsula country, including rising separatist sentiment in the south.

(Writing by Martina Fuchs and Joseph Logan; Editing by Peter Graff)