KABUL, Afghanistan (AP) — Militants staged two deadly suicide attacks today to mark the first full day of U.S. Defense Secretary Chuck Hagel's visit to Afghanistan, a fresh reminder that insurgents continue to fight and challenges remain as the U.S.-led NATO force hands over the country's security to the Afghans.

A suicide bomber on a bicycle struck outside the Afghan Defense Ministry early this morning, and about a half hour later, another suicide bomber attacked a police checkpoint in Khost city, the capital of Khost province in eastern Afghanistan.

Nine people were killed in the bombing at the ministry, and an Afghan policeman and eight civilians, who were mostly children, died in the blast in Khost, said provincial spokesman Baryalai Wakman.

"This attack was a message to him," Taliban spokesman Zabiullah Mujahid said of Hagel, in an email to reporters about the defense ministry attack.

Hagel was nowhere near the blasts, but heard them across the city. He told reporters traveling with him that he wasn't sure what it was when he heard the explosion.

"We're in a war zone. I've been in war, so 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 U.S. was going to continue to work with its allies to ensure that the Afghan people have the ability to develop their own country and democracy.

Hagel's first visit to Kabul as Pentagon chief comes as the U.S. and Afghanistan grapple with a number of disputes, from the aborted handover of a main detention facility — canceled at the last moment late Friday as a deal for the transfer broke down — to Afghan President Hamid Karzai's demand that U.S. special operations forces withdraw from Wardak province just outside Kabul over allegations of abuse.

The prison transfer, originally slated for 2009, has been repeatedly delayed because of disputes between the U.S. and Afghan governments about whether all detainees should have the right to a trial and who will have the ultimate authority over the release of prisoners the U.S. considers a threat.

The Afghan government has maintained that it needs full control over which prisoners are released as a matter of national sovereignty. The issue has threatened to undermine ongoing negotiations for a bilateral security agreement that would govern the presence of U.S. forces in Afghanistan after the current combat mission ends in 2014.

U.S. military officials said Saturday's transfer ceremony was canceled because they could not finalize the agreement with the Afghans, but did not provide details. Afghan officials were less forthcoming.

"The ceremony is not happening today," Defense Ministry spokesman Gen. Mohammad Zahir Azimi said, without elaborating.