BEIRUT, Lebanon — Two important Syrian military bases in Idlib Province, under rebel siege for two years, fell on Monday to insurgents led by the Nusra Front, the Islamist militant group aligned with Al Qaeda, fighters and activist groups monitoring the conflict reported.

The Nusra fighters appeared to have achieved the victory partly by using American TOW antitank missiles they had captured from more moderate antigovernment fighters backed by the United States, another sign that those fighters are struggling to regain the initiative from extremist groups.

In what seemed like a significant strategic loss for Syria’s military, its soldiers abandoned the bases in the country’s northwest, Wadi Deif and Hamidiyeh, after intense fighting that began Sunday morning. Activists said the attackers had seized 13 checkpoints around Wadi Deif and seven around Hamidiyeh, and by early Monday afternoon they had control of both bases.

According to the Syrian Observatory for Human Rights, a group that monitors the fighting through a network of contacts on the ground, the Nusra-led attackers captured at least 15 Syrian soldiers. It said at least 31 soldiers and 12 insurgents had been killed in the assaults.