Olga Rudenko and Jim Michaels

USA TODAY

KIEV, Ukraine — Two Ukrainian attack jets were shot down Wednesday over territory held by Russian-backed separatists in eastern Ukraine, the Ukrainian Defense Ministry said.

The Ukrainian government said the planes were downed by missiles fired from Russia, raising questions about whether Ukrainian separatists who had crossed the border were responsible or whether Russians fired the weapons.

Whatever the answer, the incident highlighted the growing conflict in eastern Ukraine between separatists who want autonomy for the region, where many ethnic Russians live, and the national government, which is trying to seize back control of the territory.

"According to the preliminary information we have, the missiles were fired from the territory of Russia," Andriy Lysenko, spokesman for Ukraine's National Council for Security and Defense told a news conference Wednesday.

He said the planes were flying at 17,000 feet when they were struck. "Such altitude is too high to be reached by portable air-defense systems," Lysenko said, referring to field weapons used by the insurgents. "It can be reached only by heavy missile complexes."

The attack and Lysenko's suggestion of Russian involvement come a day after senior U.S. intelligence officials warned that Russia was continuing to supply rebels with arms and training despite U.S. evidence that separatists used Russian weaponry to shoot down Malaysia Airlines Flight 17 last week, killing all 298 aboard. The separatists and Russia blame Ukraine's military for downing the Boeing 777.

The defense ministry said on its Facebook page that the jets were among four Russian-built fighters that were returning to base after providing air support for Ukrainian troops near the border.

The pilots of both jets ejected from the aircraft, the ministry said. Their condition was not immediately reported.

Ukraine's parliament approved a presidential decree Wednesday on a third mobilization of men to be called up to serve in the army. The first two mobilizations took place on March 17 and May 6.

As of Tuesday, 280 troops have died in the conflict and 780 have been wounded, according to Defense Minister Valeriy Geletey. The national government has vowed to regain control of key cities in eastern Ukraine now held by the separatists.

Three senior U.S. intelligence officials who briefed reporters Tuesday said rebels have shot down about a dozen aircraft over the past several months. The rebels stepped up their attacks on Ukrainian planes after its military had begun to make progress against the rebels, according to the officials, who shared intelligence findings on condition that they not be identified by name.

The Ukraine military's ability to attack rebels from the air and move troops quickly by helicopter provides an advantage over the rebels.

Defense Ministry spokesman Oleksiy Dmitrashkovsky said the Sukhoi-25 attack aircraft were downed in an area called Savur Mogila in the Shaktersky region near the Russian border.

The Sukhoi-25 are typically used for supporting ground troops. Many of the aircraft downed by rebels were attacked by ground fire.

The Malaysia plane was flying above 33,000 feet when it was downed by a sophisticated SA-11 surface-to-air missile fired from rebel-held territory, according to evidence the U.S. intelligence officials described. They said Russia provided the missile system and training on how to use it.

The site of Wednesday's attack is only a few miles from where the Malaysia Airlines jetliner crashed.

The U.S. military is supplying Ukraine's military with a $33 million package of "non-lethal" aid, including radios, body armor, first aid kits and night-vision goggles, the Pentagon said.

Ukraine's military has asked for arms and ammunition, but for now the Pentagon said it was continuing to focus on the non-lethal aid.

In addition, the Pentagon in coming weeks is sending a second assessment team to Ukraine to examine ways to provide long-term assistance to Ukraine's military. The plans for the assessment team were in the works before the downing of the Malaysia Airlines flight.

Michaels reported from Washington. Contributing: Doug Stanglin; Associated Press