Explosions in the Afghan capital Kabul killed at least 26 people on Monday, including nine journalists who had arrived to report on an initial blast and were apparently targeted by a suicide bomber, officials said.

The attacks, a week after 60 people were killed as they waited at a voter registration center in the city, underlined mounting insecurity despite repeated government pledges to tighten defences.

Hours after the attack in Kabul, a suicide bomber in a vehicle attacked a foreign military convoy in the southern province of Kandahar, killing 11 children studying in a nearby religious school, police said.

"These attacks caused untold human suffering to Afghan families," said Tadamichi Yamamoto, the top U.N. official in Afghanistan.

"I am furthermore outraged by the attack which appears to have deliberately targeted journalists," he said in a statement.

The attacks in rapid succession were a grim reminder of the strength of both the Taliban and Islamic State's emerging Afghanistan branch to wreak violence despite stepped up US air attacks under Trump's new policy for the 16-year-old war.

Taliban militants, fighting to restore their version of strict Islamic law to Afghanistan, announced their usual spring offensive last week and there has been heavy fighting in several areas of the country since.

Islamic State claimed responsibility for the two blasts in the capital, which killed at least 26 people, including four police, and wounded 49 seriously, Hashmat Stanekzai, a senior police official, said.

The Afghan Journalists Safety Committee (AFJSC) said nine journalists were killed in Kabul, the worst toll for media workers in a single attack in the country.

Najib Danish, a spokesman for the interior ministry, said a suicide bomber appeared to have posed as a media worker and blew himself up where reporters and rescue workers had gathered.

"We know that a suicide bomber pretended to be a reporter. He showed his press card and stood among journalists before blowing himself up," Danish told Reuters.