KANDAHAR, Afghanistan -- Militants killed six Americans, including a young female diplomat, and an Afghan doctor Saturday in a pair of attacks in Afghanistan on Saturday. It was the deadliest day for the United States in the war in eight months.

The violence - hours after the U.S. military's top officer arrived for consultations with Afghan and U.S.-led coalition officials - illustrates the instability plaguing the nation as foreign forces work to pull nearly all their combat troops out of the country by the end of 2014.

The attacks came just days after insurgents stormed a courthouse, killing more than 46 people in one of the deadliest attacks of the war, now in its 12th year.

The three U.S. service members, two U.S. civilians and the doctor were killed when the group was struck by an explosion while traveling to donate books to students in a school in the south, officials and the State Department said.

In a statement, Secretary of State John Kerry said the Americans included a department of defense civilian and the foreign service officer.

"She tragically gave her young life working to give young Afghans the opportunity to have a better future," Kerry said. "We also honor the U.S. troops and Department of Defense civilian who lost their lives, and the Afghan civilians who were killed today as they worked to improve the nation they love."

Officials said the explosion occurred just as a coalition convoy drove past a caravan of vehicles carrying the governor of Zabul province to the same event.

Another American civilian was killed in a separate insurgent attack in eastern Afghanistan, the U.S. military said in a statement.

It was the deadliest day for Americans since Aug. 16, when seven American service members were killed in two attacks in Kandahar province, the birthplace of the Taliban insurgency. Six were killed when their helicopter was shot down by insurgents and one soldier died in a roadside bomb explosion.

The latest attacks occurred just hours after U.S. Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, landed in Afghanistan for a visit aimed at assessing the level of training that American troops can provide to Afghan security forces after international combat forces complete their withdrawal.

A U.S. official who spoke on condition of anonymity said several other Americans and Afghans, possibly as many as nine, were wounded. The State Department said four of their staff were wounded, one critically.

Taliban spokesman Qari Yousef Ahmadi claimed responsibility for the attack in Zabul and said the bomber was seeking to target either a coalition convoy or the governor.

"We were waiting for one of them," Ahmadi said in a telephone interview with The Associated Press. "It was our good luck that both appeared at the same time."

The deaths bring the number of foreign military troops killed this year to 30, including 22 Americans. A total of six foreign civilians have died in Afghanistan so far this year, according to an AP count.

Provincial Gov. Mohammad Ashraf Nasery, who survived the attack in Qalat, said the explosion occurred in front of a hospital and a coalition base housing a provincial reconstruction team, or PRT. International civilian and military workers at the PRT train Afghan government officials and help with local development projects.

Nasery said the car bomb exploded as his convoy was passing the hospital. He said the doctor was killed, and two of his bodyguards and a student from the school were wounded.

"The governor's convoy was at the gate of the school at the same time the (coalition) convoy came out from the PRT," said provincial police chief Gen. Ghulam Sakhi Rooghlawanay. "The suicide bomber blew himself up between the two convoys."

Nasery said he thought his convoy was the intended target.

"I'm safe and healthy," he told the AP in a telephone interview.

Insurgents have stepped up attacks around the country in recent weeks as Afghanistan enters what could be one of the most critical periods following the U.S. invasion in late 2001 that ousted the Taliban.

The majority of U.S. and coalition forces are expected to begin a significant drawdown in the latter part of this year, leaving Afghan forces in charge of security across the country within months. Afghanistan also is gearing up for a presidential election next spring, and the Taliban have not yet accepted an offer to engage in peace talks in the Gulf state of Qatar.

There currently are about 100,000 international troops in Afghanistan, including 66,000 from the United States. The U.S. troop total is scheduled to drop to about 32,000 by early next year, with the bulk of the decline occurring during the winter months.

While there has been no final decision on the size of the post-2014 force, U.S. and NATO leaders say they are considering a range of between 8,000 and 12,000 - most of them trainers and advisers.

The Taliban have already sought to disrupt the political process as Afghanistan's various ethnic groups prepare to field candidates to run in the presidential elections. President Hamid Karzai is banned by the constitution from seeking a third term.

The Taliban have increasingly targeted Afghan government officials in recent attacks, including an assault on Wednesday on a courthouse and government offices in western Farah province. Forty-six people were killed, including two judges, six prosecutors, administration officers and cleaners working at the site.

The Taliban have said civilians working for the government or the coalition are legitimate targets, despite a warning from the United Nations that such killings may violate international law.

They also have been staging complex attacks in Kabul and other urban areas. On March 14, the Afghan intelligence service seized a massive truck bomb packed with 7,257 kilograms (8 tons) of explosives on the eastern outskirts of Kabul. The truck apparently was going to be used in an attack on a NATO facility in the capital.

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