A controversial law in Afghanistan that includes a provision making it illegal for a Shia Muslim woman to refuse to have sex with her husband is making it more difficult to boost NATO troop levels in Afghanistan, the secretary general of the alliance says.

"How can I defend — or how can the British government defend, the Canadian government, the Dutch government — that our boys and girls are dying there in the defence of universal values," Jaap de Hoop Scheffer said at the NATO summit at Strasbourg, France.

"And you see a law almost coming into effect… that fundamentally violates women's rights and general human rights, then I have a problem."

U.S. President Barack Obama wants to beef up the alliance's role in the conflict. But NATO allies have been reluctant to commit significant new ground troops to the war effort.

Afghan Ambassador Omar Samad has said the Afghan government is studying the law, which has sparked international outrage, to determine its status, and has pleaded for patience and understanding.

Canada and other nations have expressed strong concerns about the legislation, which would also make it illegal for a woman to leave the house without her husband's permission, or have custody of children.

"This is extremely alarming and it's troublesome for a lot of the allies," Foreign Minister Lawrence Cannon said.

But Cannon questioned whether any such Afghan law "was adopted in the legitimate way and manner" of Afghanistan's national assembly.

Cannon said two of Karzai's ministers pleaded ignorance this week, and the ambassador to Canada has been called in for an explanation.

With files from the Canadian Press