NEW YORK (AP) _ American intelligence reports say the younger brother of Afghanistan's Soviet-backed leader defected to the anti-government rebels, a newspaper reported today.

The Afghan ruler, Najibullah, and his younger brother Sidiq were said to be at odds for some time, according to The New York Times.

Both men use only one name.

Sidiq apparently joined the forces of Ahmed Shah Massoud, the rebel field commander who has led the fight against government troops and Soviet forces in the strategic Panjshir valley, said the Times, quoting American intelligence reports and unidentified foreign diplomats.

Sidiq, also known as Sidiqullah, is believed to have joined the Jamiat-i- Islami party, one of the largest opposition groups, the newspaper said. Massoud heads the party, one of a seven-party coalition.

Jamiat-i-Islami has said nothing publicly about Sidiq.

Najibullah has campaigned in recent months for a ''national reconciliation,'' and the defection of his brother would be an embarrassing setback for the campaign, the Times said.

An estimated 115,000 Soviets are stationed in Afghanistan, embroiled in civil war since 1979.