Islamist fighters have seized 17 checkpoints from Syrian regime forces in clashes around the city of Idlib that have cost at least 68 lives in 48 hours, a monitor said Thursday.

"The government and its supporting forces have lost 17 checkpoints and regime locations to the Al-Nusra Front and other Islamist groups in and around Idlib" in the country's northwest, said the Syrian Observatory for Human Rights. 

At least 18 people were killed Thursday as supporters of President Abedrabbo Mansour Hadi clashed with Shiite rebels advancing on Aden, where the internationally backed leader has sought refuge, a local official said.

The clashes erupted in Huta, capital of the southern Lahj province, when the pro-Hadi "popular committees" were resisting the rebel advance, the official said.

Saudi Arabia halted flights at seven airports near the Yemeni border on Thursday, as it launched air strikes against Huthi rebels in its southern neighbor, the civil aviation department said.

"The General Authority of Civil Aviation of Saudi Arabia announced a temporary suspension of international and domestic flights to and from airports in the south of the kingdom," from dawn on Thursday, the department said in a statement.

Uganda said Thursday it was boosting security over threats by Somalia's Shebab militants, hours after the U.S. embassy in Kampala warned its citizens of a possible imminent terror attack.

In an emergency warning issued late Wednesday, the U.S. mission in Kampala said it had "received information of possible terrorist threats to locations where Westerners, including U.S. citizens, congregate in Kampala, and that an attack may take place soon".

Britain will begin training Syrian "moderate opposition forces" in Turkey and other regional countries within weeks as part of its efforts to fight Islamic State militants, Defense Secretary Michael Fallon announced on Thursday.

"Defeating ISIL ultimately lies with local forces and we are helping to create effective ground forces in Syria, as well as in Iraq, so they can take the fight to ISIL," Fallon said, using one of the acronyms used to refer to the IS group.

Pirates have seized an Iranian vessel fishing illegally off the coast of Somalia in the first successful hijacking in almost three years, officials said Thursday, raising fears that the scourge of Somali piracy may be returning.

"An Iranian vessel, fishing illegally in Somali waters, was hijacked earlier this week," said Alan Cole of the United Nations Office on Drugs and Crime (UNODC).

Morocco has joined the Saudi-led military coalition that has intervened to support Yemeni President Abedrabbo Mansour Hadi in his fight against Iran-backed Shiite rebels, the foreign ministry said Thursday.

Rabat has "put at the disposition" of the coalition Moroccan warplanes already based in the United Arab Emirates, from which they have been participating in U.S.-led strikes against the Islamic State group, a statement said.

Sudan is providing ground troops as well as warplanes for a Saudi-led military intervention against Shiite rebels in Yemen, the defense minister said on Thursday.

Saudi warplanes launched strikes on the Huthi rebels on Thursday in a bid to stop their advance on embattled President Abedrabbo Mansur Hadi's refuge in the main southern city of Aden.

Pakistan is examining a request from Saudi Arabia to join a coalition to defend Yemen's president against advancing rebels threatening his stronghold in Aden, Islamabad said Thursday.

The Gulf kingdom has begun air strikes against Shiite Huthi rebels and the Saudi ambassador in Washington said a coalition of 10 countries, including Pakistan, was being formed to protect the Yemeni government.

More than 1,000 civilians have been killed in Boko Haram attacks in the first three months of this year, Human Rights Watch said on Thursday.

"Each week that passes we learn of more brutal Boko Haram abuses against civilians," the group's Nigeria researcher, Mausi Segun, said in an emailed statement.