The top U.S. commander for NATO said Thursday that America needs better intelligence on the ground in Ukraine, but that it appears Russian forces have used a recent lull in fighting to reposition for another offensive.

Gen. Philip Breedlove, commander of NATO forces in Europe, told the Senate Armed Services Committee, that the situation in Ukraine is volatile and fragile and urged Congress to bolster U.S. intelligence capabilities to better understand President Vladimir Putin's intent in the region.

"Russian military operations over the past year in Ukraine, and the region more broadly, have underscored that there are critical gaps in our collection and analysis," Breedlove said. "Some Russian military exercises have caught us by surprise and our textured feel for Russian involvement on the ground in Ukraine has been quite limited."

He said the number of Russia intelligence experts has dwindled since the Cold War and intelligence assets of all kind have been shifted to the wars in Iraq and Afghanistan.

"We cannot be fully certain what Russia will do next and we cannot fully grasp Putin's intent. What we can do is learn from his actions," Breedlove said. "What we do see suggests growing Russian capabilities, significant military modernization and an ambitious strategic intent."

"Russian forces used the opportunities presented by the recent lull in fighting to reset and reposition while protecting their gains," he said. "Many of their actions are consistent with preparations for another offensive."

The United States now sees the Ukrainian rebels as a Russian force.

American officials briefed on intelligence from the region say Russia has significantly deepened its command and control of the militants in eastern Ukraine in recent months, leading the U.S. to quietly introduce a new term: "combined Russian-separatist forces." The State Department used the expression three times in a single statement last week, lambasting Moscow and the insurgents for a series of cease-fire violations in Ukraine.

The shift in U.S. perceptions could have wide-ranging ramifications, even if the Obama administration has cited close linkages between the pro-Russian separatists and Putin's government in Moscow since violence flared up in Ukraine a year ago.

By describing them as an integrated force in the east of the country, the U.S. is putting greater responsibility on Russia for the continued fighting. That will make it harder for Russia to persuade the U.S. and Europe to scale back sanctions that are hurting its economy, and for Washington and Moscow to partner on unrelated matters from nuclear nonproliferation to counterterrorism.

U.S. intelligence agencies signed off on the new language last week, after what officials outlined as increasing evidence of the Russians and separatists working together, training together and operating under a joint command structure that ultimately answers to Russia. The officials weren't authorized to be quoted by name and demanded anonymity.

Some of that evidence was presented in a statement released by State Department spokeswoman Marie Harf on April 22 after Secretary of State John Kerry raised his concerns by telephone with Russian Foreign Minister Sergey Lavrov.

Harf spoke of Russia's deployment of air defense systems closer to the front lines, increased troop levels near Kharkov, Ukraine's second largest city, and intensified training sessions involving the use of Russian drones. She called the unmanned aerial vehicles "an unmistakable sign of Russia's presence."

The uprising began after protesters chased Ukraine's pro-Russia president out of power and Moscow responded by annexing the Ukrainian region of Crimea. The insurgency started with miners, farmers and others without military training rebelling against the new government, and quickly expanded. More than 6,000 people have died and a million have been displaced by the conflict.

Russia's air defense concentration in eastern Ukraine is now at its highest level since August, the U.S. says. Russia has more ground forces at the border than at any point since October. These developments and others have American officials fearful that Moscow and the separatists may be planning an offensive in the coming weeks.

"War could start at any moment," Ukrainian President Petro Poroshenko said this week.

Recent fighting has been concentrated near the Black Sea port of Mariupol. Ukraine's government still controls the territory, but a separatist takeover would establish a land bridge between mainland Russia and Crimea.

Associated Press reporting in eastern Ukraine this spring showed Russia expanding its training of separatist fighters to improve their capabilities to operate sophisticated Russian weaponry and defend territory. At the same time, Russia has reduced the number of its troops deployed in Ukraine.

The shift appears designed to minimize Russia's visible military presence while it seeks to persuade the West to lift economic sanctions.

Yet Russian troops have been a "permanent feature of the conflict," said Igor Sutyagin, a London-based Russia scholar.

Russian forces in Ukraine, he said, peaked at about 9,000 in late February. He calculated the estimation on sightings of weaponry and postings by soldiers on social media. Several hundred Russian military trainers are likely in eastern Ukraine right now, he said.

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Associated Press writer Nataliya Vasilyeva in Moscow and Deb Riechmann in Washington contributed to this report.