With cargo ships poised to launch a desperately needed aid operation in embattled Yemen, 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.

Earlier Tuesday, Iran said it was sending warships to protect an Iranian aid ship steaming toward a Yemeni port held by the rebel fighters, the Iranian state news agency said. The navy escort was denounced by the Pentagon and Saudi Arabia as unnecessary, and raised the possibility of a confrontation near the strategic Bab el-Mandab strait in the Gulf of Aden.

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 metric tons (330 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 250,000 liters (66,050 gallons) of fuel and supplies for other humanitarian agencies. A second vessel is ready to dock with an additional 120,000 liters (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 Houthis military — something the rebels and the Islamic Republic deny.

On Tuesday, Iran said it was sending an aid ship carrying food, medicine, tents and blankets, as well as rescue workers, journalists and peace activists that was expected to arrive next week in the port of Hodeida, which was seized by the Houthis and their allies last fall. The official IRNA news agency said it was being protected Iranian warships currently patrolling in the Gulf of Aden.

U.S. Army Col. Steve Warren in Washington said the U.S. was monitoring the Iranian vessel. He warned that it would not be helpful if Iran is "planning some sort of stunt," recommending that Tehran instead send the vessel to Djibouti, where humanitarian efforts for Yemen are being coordinated.

Saudi Brig. Gen Ahmed Asiri said no ship would be permitted to reach Yemen unless there was prior coordination with the coalition. Speaking to the Saudi-owned satellite channel Al-Arabiya, he added that if Iran wants to deliver humanitarian aid to Yemen it should be done through the United Nations.

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.

"The Houthis and other armed groups using child soldiers in Yemen should immediately stop recruiting children, including 'volunteers,' and release all children in their ranks," it said.

Security officials said Monday the Houthis were recruiting boys as young as 15 to fight in Saada, a rebel stronghold north of the capital, against Sunni tribesmen trying to enter the province along the border with Saudi Arabia.

Tuesday's airstrikes came one day after the coalition pounded a mountainside on the northeastern edge of Sanaa, hitting arms and ammunition depots. The Houthi-held Health Ministry said that preliminary figures show those airstrikes killed 69 people and wounded more than 100, mostly civilians.