Hundreds of Sudanese troops arrived in Yemen's southern port city of Aden on Saturday, the first batch of an expected 10,000 reinforcements for the Saudi-led coalition fighting the country's Shia Houthi rebels, security officials said.

Their mission is to secure Aden, which has seen an uptick in drive-by shootings of pro-government troop leaders and officials as extremists became more entrenched in the city in recent weeks, the pro-government security officials said.

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Yemen's fighting pits the Houthis and allied army units against forces loyal to the coalition-backed, internationally recognized government as well as southern separatists and other militants.

The latest assassination was of an Emirati officer in Aden's Mansoura neighbourhood on Friday, killed by gunmen on a motorcycle, officials said. The United Arab Emirates is part of the Saudi-led coalition, which has been pounding rebel positions since March.

Attack unclaimed

Although the attack, like several others, went unclaimed, the officials said they suspect Sunni extremists, who they say have made land grabs, exploiting the chaos engulfing the Arab world's poorest country. Yemen's al-Qaeda, viewed by Washington as the terror network's most dangerous affiliate, is known to have used motorcycles in previous assassinations.

Earlier Saturday, al-Qaeda militants set up security checkpoints and began enforcing sex segregation at the sole college in Zinjibar, the provincial capital of Abyan, neutral and pro-government security officials there said.

"First they took Mukalla and then Zinjibar. We are all worried Aden may be next," one pro-government security official told The Associated Press.

Yemen's al-Qaeda branch overran Mukalla, the capital of sprawling Hadramawt province, in April. They have since gender-segregated public spaces there and publicly killed and flogged people, including on charges of "witchcraft," Mukalla residents told The Associated Press last week.

Friendly fire

Also Saturday, airstrikes from the Saudi-led coalition targeting Houthi rebels mistakenly struck a pro-government military encampment, killing at least 20 fighters and wounding another 20 in the latest instance of friendly fire in the anti-rebel camp, security officials said.

The fighters had just wrested the encampment from the Houthis in the southern Taiz province when airstrikes hit them, pro-government security officials said.

"They thought the Houthis were still there," one pro-government security official told The Associated Press.

Ground commanders have repeatedly complained of slow communication with military leadership in Riyadh, the officials added.

In the massive desert province of Jawf, Saudi airstrikes killed 13 Houthis, neutral security officials there said. The strikes are part of a plan to seize the northern province in order to advance on the Houthi heartland of Sadaa, pro-government officials said.

All officials and witnesses spoke on condition of anonymity because they are not authorized to brief reporters or fear reprisals.