TEHRAN, Iran - Iran's navy said Tuesday it will protect an aid ship traveling to Yemen where a five-day humanitarian cease-fire is set to begin between a Saudi-led coalition and Shiite rebels and their allies. The U.S. quickly warned against the move, which comes amid heightened tensions after the Islamic Republic seized a cargo ship in recent weeks.

The state-run IRNA news agency quoted Iranian Admiral Hossein Azad as saying that the 34th naval group "is present in the Gulf of Aden and Bab al-Mandab strait and has been given the specific mission of protecting the humanitarian aid ship." That naval group includes the destroyer Alborz and logistic ship Bushehr, which are on a 90-day anti-piracy assignment in the region.

Iranian state television reported Tuesday that the ship, the Iran Shahed, carried food, medicine, tents and blankets, as well as reporters, rescue workers and peace activists. It said the ship is expected to arrive at Yemen's port city of Hodeida next week.

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In Washington, U.S. Army Col. Steve Warren said the U.S. is monitoring the cargo ship and he warned that it would not be helpful if Iran is "planning some sort of stunt." He said using Iranian warships to accompany the ship is not necessary and Iran should just send the cargo vessel to Djibouti, where humanitarian efforts for Yemen are being coordinated.

There are some six U.S. warships already in the region around Yemen, including in the Gulf of Aden.

The U.S. and other Western countries have accused Iran of militarily backing the Yemeni rebels, known as Houthis. Both the rebels and the Islamic Republic deny that.

Meanwhile, a five-day humanitarian cease-fire began Tuesday night, just hours after Saudi-led coalition warplanes struck against Shiite rebels and their allies.

There were reports of continued ground fighting in some areas, with security officials and witnesses saying fierce combat broke out about a half hour after the cease-fire began when rebels tried to storm the southern city of Dhale, firing tank shells, rockets and mortars. But no airstrikes by the Saudi-led coalition battling the rebels were reported.

The officials and tribal leaders also accused the rebels and their allies of reinforcing their positions.

The truce will test the adversaries' desire to enter into peace talks to try to end the fighting that has killed hundreds of civilians since March. Both sides say they are ready to respond with violence if their opponent breaks the cease-fire.

The Saudi-led strikes in Yemen came to a halt shortly before the new U.N. envoy, Ismail Ould Cheikh Ahmed, flew into the capital, Sanaa, on his first official visit to the country. He told reporters he planned to meet with the warring parties, including the rebels known as Houthis, and ensure that the cease-fire holds.

"We will discuss the humanitarian truce and the Yemeni parties' return to the negotiating table," he said.

The cease-fire is meant to help ease the suffering of civilians in Yemen, the Arab world's poorest country.

Before the cease-fire began, security officials said airstrikes overnight, at dawn and during the morning hours Tuesday hit weapons depots and other military facilities north and south of Sanaa, a sprawling city of some 4 million people. The military air base that is part of the capital's international airport also was targeted.

Ten strikes hit Sanaa from dawn until about noon, said the officials, speaking on condition of anonymity because they were not authorized to talk to journalists.

Fierce fighting between the rebels and forces loyal to exiled President Abed Rabbo Mansour Hadi also raged in the strategic city of Taiz, southwest of the capital. The rebels and their allies shelled residential areas, with one shell hitting a bus, killing nine people and wounding 40, officials said. A coalition airstrike targeted the city's al-Qahira castle, where the shelling came from, they said.

The Saudis and their allies are seeking the restoration of Hadi, who fled the country in March.

Meanwhile, a suspected U.S. drone strike hit a car, killing three al-Qaida fighters near Shabwa province, an area where the extremist group had been sending reinforcements. Witnesses and tribal elders said the vehicle burned and set off secondary explosions from ordnance it had been carrying. They spoke on condition of anonymity out of fear for their safety.

The conflict has killed more than 1,400 people - many of them civilians - since March 19, according to the U.N. The country of some 25 million people has endured shortages of food, water, medicine and electricity as a result of a Saudi-led naval, air and land blockade.

Anticipating the truce, the U.N. refugee agency said it plans to airlift 300 tons of sleeping mats, blankets, kitchen sets and plastic sheeting from stockpiles in Dubai.

The airlift was part of what it called a "larger aid mobilization underway for a quarter of a million people." The agency also will attempt to distribute aid already stored in Yemen and assess the needs for areas that have been difficult to reach.

Separately, the U.N. World Food Program said it was ready to provide emergency food rations to more than 750,000 people. A vessel chartered by the agency arrived in the Red Sea port of Hodeida on Saturday, carrying 66,050 gallons of fuel and supplies for other humanitarian agencies. A second vessel is ready to dock with an additional 31,700 gallons of fuel.

The Houthis and forces loyal to ousted President Ali Abdullah Saleh overran Sanaa and much of northern Yemen late last year and have been on the offensive in the south. Western nations say Shiite power Iran supports the Houthi's militarily - something the rebels and the Islamic Republic deny.

Also Tuesday, Human Rights Watch said the Houthis have intensified the recruitment of children in the conflict in violation of international law. Since the rebels seized Sanaa in September, it said, the Houthis increasingly used children as scouts, guards, runners and fighters, with some children being wounded or killed.