KABUL: US Defense Secretary Chuck Hagel arrived in Afghanistan on Saturday for his first visit since taking the post, but his trip was marred by insurgent suicide bombings and last-minute delays in a long-awaited deal to transfer a US detention center to Afghan control.  

Nineteen Afghans were killed _ including eight children _ in two separate suicide attacks in Kabul and in the eastern Khost province Saturday.

A US contractor was killed and four soldiers injured when attackers thought to be Afghan soldiers stormed their base and opened fire Friday, just hours before Hagel arrived. ''This attack was a message to him,'' Taliban spokesman Zabiullah Mujahid said in an email to reporters about one of the bombings, which was outside the country's Defense Ministry in Kabul.

Afghan President Hamid Karzai, meanwhile, was standing by a demand that US special operations forces leave a province neighbouring Kabul by Monday for alleged abuses of Afghan civilians _ charges US officials deny. And a handover ceremony scheduled for Saturday of a US detention facility also fell through, when US and Afghan officials ran into last-minute disagreements.

The barrage of crises heralding Hagel's arrival illustrates the complex minefield of diplomatic and military issues facing the US-led Nato force, and the Obama administration, as they hand over the country's security to Afghans ahead of the 2014 deadline for the end of Nato combat operations.

The violence also shows the Afghan security forces' struggle to contain the Taliban as Nato troops slowly withdraw, even as Afghan President Hamid Karzai argues for his government to have more control over Afghanistan's security.

Nato officials see the weekend violence as part of the Taliban's coming campaign for the spring fighting season. ''There's a series of attacks that have started as the snow is thawing. We had a potential insider attack yesterday ... and there's been a number of attacks on the border,'' said Brigadier Adam Findlay, Nato's deputy chief of staff of operations, in an interview with The Associated Press Saturday.

In the first attack Saturday, a suicide bomber on a bicycle struck outside the Afghan Defense Ministry around 9 a.m. local time, just as employees were arriving for work. About a half hour later, another suicide bomber targeted a joint Nato and Afghan patrol near a police checkpoint in Khost city, the capital of Khost province in eastern Afghanistan, said provincial spokesman Baryalai Wakman.

Nine Afghan civilians were killed in the bombing at the ministry and 14 wounded, and two Afghan policemen and eight children died in the blast in Khost while another two Afghan civilians were wounded, according to a statement from Karzai's office.

Karzai condemned the bombings, calling them un-Islamic. ''The perpetrators of such attacks are cowards who are killing innocent children at the orders of foreigners,'' he said in a statement emailed to reporters. Karzai usually uses the term ''foreigners'' to refer to Pakistan, which he blames for failing to crack down on Taliban militants who take sanctuary there.

Hagel was nowhere near the Kabul blast, but heard it across the city. He told reporters travelling with him that he wasn't sure what it was. ''We're in a war zone. I've been in war, so you shouldn't be surprised when a bomb goes off or there's an explosion,'' said Hagel, a Vietnam War veteran.

Asked what his message to the Taliban would be, he said that the US was going to continue to work with its allies to insure that the Afghan people have the ability to develop their own country and democracy.

In addition, a suspected insider attack by Afghan soldiers against their US allies occurred in Kapisa province in eastern Afghanistan several hours before Hagel arrived Friday. Three men presumed to be Afghan soldiers forced their way onto a US base and opened fire, killing one US civilian contractor and wounding four US soldiers, according to a senior US military official.