WASHINGTON — Intensifying sectarian and clan violence has presented new opportunities for jihadist groups across the Middle East and raised concerns among American intelligence and counterterrorism officials that militants aligned with Al Qaeda could establish a base in Syria capable of threatening Israel and Europe.

The new signs of an energized but fragmented jihadist threat, stretching from Mali and Libya in the west to Yemen in the east, have complicated the narrative of a weakened Al Qaeda that President Obama offered in May in a landmark speech heralding the end of the war on terrorism. The leaders of the Senate and House intelligence committees, Senator Dianne Feinstein of California and Representative Mike Rogers of Michigan, raised warnings in an interview on CNN on Sunday when they said that Americans were “not safer” from terrorist attacks than they were in 2011.

The concerns are based in part on messages relayed this year by Ayman al-Zawahri, Al Qaeda’s overall leader, indicating that he views Syria — where the number of jihadist rebels and foreign fighters is steadily rising — as a promising staging ground.

Some analysts and American officials say the chaos there could force the Obama administration to take a more active role to stave off potential threats among the opposition groups fighting against the government of President Bashar al-Assad. But striking at jihadist groups in Syria would pose formidable political, military and legal obstacles, and could come at the cost of some kind of accommodation — even if only temporary or tactical — with Mr. Assad’s brutal but secular government, analysts say.