AL UDEID AIR BASE, Qatar — Russian fighter jets have flown dangerously close to American warplanes in eastern Syria over the past month — including one near-collision — in what American officials say is a pattern of daily Russian violations of an agreement to separate rival forces converging on the last main pocket of Islamic State militants in the country.

In one instance, two Air Force A-10 attack planes flying east of the Euphrates River nearly collided head-on with a Russian Su-24 Fencer just 300 feet away — a knife’s edge when all the planes were streaking at more than 350 miles per hour. The A-10s swerved to avoid the Russian aircraft, which was supposed to fly only west of the Euphrates. Other Russian planes have flown within striking distance or directly over allied ground forces for up to 30 minutes, escalating tensions and the risk of a shootdown, American officials said.

Since American and Russian commanders agreed last month to fly on opposite sides of a 45-mile stretch of the Euphrates to prevent accidents in eastern Syria’s increasingly congested skies, Russian warplanes have violated that deal half a dozen times a day, according to American commanders. They say it is an effort by Moscow to test American resolve, bait Air Force pilots into reacting rashly, and help the Syrian Army solidify territorial gains ahead of diplomatic talks aimed at resolving the country’s nearly seven-year-old war.

“There’s risk there,” Lt. Gen. Jeffrey L. Harrigian, the air commander for Syria and Iraq, said of the violations, in an interview at his headquarters at the Al Udeid Air Base in Qatar. “Their desire is to set this up for the end state for Syria. We’ve got to be cleareyed. The Russians are here to support the Syrian government.”