Saudi-led coalition airstrikes came to a halt in Yemen early Monday after a five-day humanitarian truce went into effect, witnesses and security officials said.

However, ground fighting broke out almost immediately in the restive city of Taiz following random shelling by Shiite Houthi rebels in three neighborhoods, they said.

Security officials said ground fighting has also erupted in Marib province and in the area surrounding the strategic al-Anad military base in Lahj province.

Random shelling by Houthis and their allies hit northern and western areas of the port city of Aden after the cease fire, security officials and witnesses said.

The Saudi-led and U.S.-backed coalition of mainly Gulf Arab countries has been waging an air campaign since March against the Iran-supported rebels, who control most of northern Yemen and the capital, Sanaa.

The pause declared by the Saudi-led coalition began at 11:59 p.m. (2059 GMT, 4:59 p.m. EDT) Sunday. It is intended to help allow humanitarian aid to be delivered to ease the suffering of civilians in the Arab world's poorest country.

The coalition made the unexpected announcement about the humanitarian pause on Saturday. The statement, carried on Saudi state media, said the coalition will cease military operations, but that it will respond should Houthi rebels or their allies conduct any military actions or movements.

The rebels, known as Houthis, have expressed doubt over the truce. One Houthi official said it will likely mark "the beginning of a new war." Mohammed Ali Al-Houthi, the head of the Houthi's Revolutionary Council, said Sunday that the group had not received official notification of the truce from the United Nations.

Two previous humanitarian truces in Yemen did not hold.

U.N. Secretary-General Ban Ki-moon welcomed the coalition's announcement of the cease-fire and urged the Houthis and other parties to suspend military operations and "maintain the humanitarian pause for the sake of all the Yemeni people," Ban's spokesman said. Ban also urged all sides "to facilitate the delivery of humanitarian assistance to all parts of Yemen."

Earlier on Sunday, Saudi-backed Yemeni troops and their allies clashed with Houthi rebels in a strategic town north of the port city of Aden, security and military officials from both sides of the conflict said.

The pro-government fighters had withdrawn from the town of Sabr earlier in the day after fierce battles with the Houthis. They returned hours later following the arrival of military reinforcements and wrested control of a large portion of the town, security officials said.

The officials said five pro-government fighters were killed and 15 wounded in the battle. Local medical officials said eight rebels were killed and 20 wounded.

The running battles in Sabr, which is on a key supply route, have lasted for more than a day after troops stormed it in their push north from Aden toward the strategic military base of Al-Anad, which is held by the rebels.

Security officials and residents of Sabr said the situation on the ground has quieted after the cease-fire took effect.

The Yemeni troops fighting in Sabr had been training since April in military camps in al-Buraiqeh, the port city west of Aden, military officials from the Saudi-led coalition said. Saudi, Emirati, Egyptian and Jordanian military advisers there have set up the camps and trained hundreds of fighters, they added.

The fighters also received over 300 armored personnel carriers from the United Arab Emirates by sea. These sophisticated carriers are largely driven by non-Yemenis, the military officials said. Two Emirati officers have been killed in battle in Yemen since the Saudi-led coalition's latest offensive, known as the "Golden Arrow," started on July 16.

Al-Buraiqeh is also home to the Yemeni Fourth Military Base, which is in charge of all military operations in Aden.

The foreign military advisers, officials said, arrived in al-Buraiqeh by sea more than a month ago and serve as intermediaries between the Yemeni troops and the coalition leadership in neighboring Saudi Arabia. They also supervise the distribution of weapons and give coordinates for coalition airstrikes, military officials said.

All the officials spoke anonymously because they are not authorized to brief reporters.