Officials in northern Afghanistan say a suicide bomb attack on a mosque has killed at least 40 people and wounded dozens of others.

Authorities say the bomber struck the mosque in Maymana, the capital of Faryab province, on Friday, at the start of the four-day Muslim holiday of Eid al-Adha. Officials say casualties included members of Afghanistan's security forces.

The explosion came as President Hamid Karzai delivered a speech in Kabul marking the Eid holiday.

There was no immediate claim of responsibility for the attack, but suicide bombings are a favorite weapon of Taliban Islamists trying to topple the Western-backed government of President Karzai.

Meanwhile, the U.S. military says it is investigating the deaths of two American service members after a man wearing an Afghan National Police uniform used his weapon against them.

Military officials said Thursday's incident occurred in Uruzgan province. It is not clear if the attacker was a member of the Afghan National Security Forces.

The incident bears the hallmark of a so-called “insider attack,” in which Afghan security forces or insurgents disguised in Afghan or coalition uniforms open fire on NATO personnel. Such attacks have escalated this year, killing more than 50 coalition troops.