Sanaa (AFP) - An Iranian naval convoy suspected of carrying weapons for Shiite rebels in Yemen has turned back, US officials said, as Saudi-led warplanes kept up air strikes on anti-government forces.

The conflict has sent tensions soaring between Sunni-ruled Saudi Arabia and Shiite Iran, which backs the Huthi rebels, raising fears Yemen could become a new front in a proxy war between Middle East powers.

Yemeni Foreign Minister Riyadh Yassin accused Tehran Thursday of trying to break a naval blockade on his country, describing the war as an "Iranian plot implemented by the Huthi militia".

A US official said Thursday the nine-ship Iranian convoy that had been heading for Yemen is "no longer on the same course".

The USS Theodore Roosevelt aircraft carrier and other American warships have deployed off Yemen's coast to track the Iranian flotilla and possibly prevent any arms deliveries to the rebels.

The flotilla included two "armed vessels," said the US official.

It was possible the Iranians "could make a turn to Yemen at any time," the official added.

Iran vehemently denies arming the rebels and has presented a peace plan to the UN calling for a ceasefire and the formation of a unity government.

On Friday, Tehran summoned the Saudi envoy to protest after warplanes allegedly turned back humanitarian aid flights headed for Yemen, whose airspace is controlled by a Saudi-led coalition.

With international pressure mounting for a political solution, UN Secretary General Ban Ki-moon announced plans to appoint Mauritanian diplomat Ismail Ould Cheikh Ahmed as his new envoy to Yemen.

The Huthis swept into the capital in September from their northern stronghold, and later advanced south on the major port of Aden, forcing President Abedrabbo Mansour Hadi to flee to Riyadh last month.

- Children living in fear -

More than 1,000 people have been killed in the fighting since late March, according to the UN, which said Friday at least 115 children were among the dead.

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"We believe that these are conservative figures," UN children's agency spokesman Christophe Boulierac told reporters in Geneva, saying at least 64 of the dead children were victims of air strikes.

The UN agency said 26 children had been killed by unexploded ordnance and mines, 19 by gunshots and three by shelling.

Another 172 children had been maimed.

"There are hundreds of thousands of children in Yemen who continue to live in the most dangerous circumstances, many waking up scared in the middle of the night to the sounds of bombing and gunfire," UNICEF representative in Yemen Julien Harneis said in a statement.

The Saudi-led coalition declared Tuesday that the first phase of its operations against the Huthis and their allies was over, but there has been no end to its air strikes.

The rebels have demanded a complete halt to the raids as a condition for UN-sponsored peace talks.

Coalition warplanes hit a camp housing rebel forces in third city Taez Friday, residents said, after a night of clashes and raids throughout the country.

Army units which remained loyal to strongman Ali Abdullah Saleh after he stepped down in 2012 following a bloody year-long uprising have provided crucial support to the Huthis.

The main southern city of Aden also came under coalition fire as clashes between Hadi supporters and rebels raged until dawn, pro-government militiamen said.

Residents in the eastern province of Marib also reported overnight air strikes and clashes between local tribesmen and rebels.

Air raids Friday struck rebel convoys, including tanks, in the southern Abyan province, leaving several dead and wounded, said pro-Hadi militiamen.

There were also strikes on nearby Daleh, local officials told AFP, adding that coalition warplanes destroyed a bridge linking the central province of Ibb and Shiite-majority Dhammar farther north to cut off rebel supplies.

- 'Immense' civilian toll -

The Saudi-led alliance says it has destroyed the Huthis' missile and air capabilities, removing the threat to neighbouring countries.

But the capital remains in rebel hands while Al-Qaeda has exploited the instability to seize more territory in the largely lawless southeast.

Saudi Arabia is also grappling with a militant threat at home.

The kingdom said Friday it had foiled a bomb plot by the Islamic State group and blamed the jihadists for shooting dead two policemen in the capital earlier this month.

The UN says millions have been affected by the Yemen conflict and are struggling to access healthcare, water, food and fuel.

"The toll on civilians has been immense," UN humanitarian coordinator for Yemen Johannes Van Der Klaauw said.

The UN's human rights agency said Friday at least 551 of the people killed were civilians.