A suicide bomber killed three coalition troops near the U.S. Embassy in Kabul on Tuesday morning, and a fourth died late Monday when an Afghan soldier opened fire on NATO forces.

The violence, the deadliest aimed at NATO troops in more than two months, underscores the continued vulnerability of international forces serving in Afghanistan.

At about 8:15 a.m., a car packed with explosives detonated near a convoy of armored vehicles in Kabul. The blast occurred as the convoy left a heavily fortified area near the U.S. Embassy and other government buildings, witnesses and Afghan officials said.

“It was a very powerful blast and happened just as the vehicles sped out of the embassy,” said Taj Mohammad, one of the witnesses.

Television footage showed mangled vehicles and coalition troops providing first aid to two soldiers lying next to the road. At least 13 Afghan civilians also were wounded, said Hashmat Stanekzai, a Kabul police spokesman.

Afghan security forces keep watch at the site of a suicide attack in Kabul on Sept. 16, 2014. A large blast ripped through central Kabul, shaking buildings and setting off the siren alert at the U.S. embassy. (Shah Marai/AFP/Getty Images)

The Taliban asserted responsibility for the attack. The assault was the deadliest for coalition forces since July 8, when five soldiers from the Czech Republic were killed during a Taliban attack on Bagram air base, north of Kabul, according to iCasualties.org, which tracks coalition casualties.

Although there has been a rapid drawdown of coalition forces this year, about 41,000 troops from 44 nations remain in Afghanistan. About three-fourths of them are Americans, and President Obama announced this summer that he plans to keep up to 10,000 U.S. troops in Afghanistan into 2015.

The coalition did not reveal the nationalities of the slain soldiers.

The blast, which could be heard for miles, occurred less than 24 hours after a coalition soldier died in an “insider attack” in western Afghanistan.

NATO officials said the soldier was shot by a man wearing an Afghan army uniform. It is thought to be the first such attack since Aug. 5, when an Afghan soldier fatally shot a two-star U.S. general.

Army Maj. Gen. Harold J. Greene was shot during a joint training exercise. Greene was the highest-ranking U.S. service member killed in the war. About a dozen other coalition soldiers, including at least five Americans, were wounded in the attack.

According to the Long War Journal, an online publication focused on counterterrorism issues, there have been 88 insider attacks since 2008, resulting in 142 coalition fatalities. Such “green on blue” attacks peaked in 2012 but have slowed dramatically over the past two years.

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In a recent interview, Afghan army spokesman Mohammad Zahir Azimi said combating the threat has been a chief priority of the country’s military.

“We are deeply concerned about this because it affects the mutual trust between soldiers,” Azimi said.

The violence targeting coalition troops this week comes as Afghans await the conclusion of an election to replace outgoing President Hamid Karzai.

Former finance minister Ashraf Ghani and former foreign minister Abdullah Abdullah have been locked in a heated dispute over the validity of the results of the June runoff. Ghani easily prevailed in that second round of voting, but Abdullah has alleged widespread fraud.

In recent days, however, both candidates have made significant progress in the formation of a national unity government, according to a Karzai spokesman and campaign officials.

The compromise deal, brokered by the United States, calls for the top vote-getter to serve as president and the second-place finisher to serve in a new chief executive position. The presidential inauguration could be held as early as next week.