— U.S. and Ukrainian officials warned Sunday that Russia may be poised to expand its territorial conquest into eastern Ukraine and beyond, with a senior NATO official saying that Moscow might even order its troops to cross Ukraine to reach Moldova.

The warnings came as Russia was finalizing its takeover of Ukrainian military bases in Crimea, the peninsula it occupied at the start of March and subsequently annexed.

Ukrainian Foreign Minister Andriy Deshchytsya, appearing on ABC’s “This Week with George Stephanopoulos,” said the prospect of war with Russia is growing.

“We don’t know what [Russian President Vladimir] Putin has in his mind and what would be his decision,” Deshchytsya said. “That’s why this situation is becoming even more explosive than it used to be a week ago.”

In Brussels, U.S. Air Force Gen. Philip M. Breedlove, the commander of U.S. and NATO forces in Europe, said Russia had assembled a large force on Ukraine’s eastern border that could be planning to head for Moldova’s separatist Transnistria region, more than 300 miles away.

Ukrainian officials have been warning for weeks that Russia is trying to provoke a conflict in eastern Ukraine, a charge that Russia denies. But Breedlove said Russian ambitions do not stop there.

“There is absolutely sufficient force postured on the eastern border of Ukraine to run to Transnistria if the decision was made to do that, and that is very worrisome,” Breedlove said.

Russia’s intent unclear

A drive into Transnistria would mark an extraordinary deepening of Russia’s military thrust into former Soviet territory and sharply escalate tensions with the West. Transnistria, a narrow strip of land about the size of Rhode Island that is wedged between the rest of Moldova and southern Ukraine, proclaimed its independence in 1990. Its population went on to vote in 2006 to seek eventual unification with Russia.

Although those moves were not recognized internationally, the region has its own constitution and currency, and pro-Russian sentiment there runs high. About 1,200 Russian troops are stationed in the territory — fewer than were in Crimea, the site of a key Russian naval base, before the current crisis began.

In Washington, a senior Defense Department official said it was “difficult to know what [Russia’s] intent is; they’re not exactly being transparent.” He spoke on the condition of anonymity because of diplomatic sensitivities.

During a conversation Thursday, Russian Defense Minister Sergei Shoigu assured U.S. Defense Secretary Chuck Hagel that Russian troops on the Ukrainian border were merely conducting a regular “spring” exercise and that Russia had no intention of sending the forces across the international line, the U.S. official said.

But at the same time, the official said, “They have enough troops close enough and, most likely, ready enough that we would have very little notice” if they decided to move farther outside Russia.

Russian news services quoted Deputy Defense Minister Anatoly Antonov as saying Sunday that Russia is complying with all international agreements on troop limits near its border with Ukraine.

In Kiev, the capital of Ukraine, members of a visiting U.S. congressional delegation said Ukrainian officials were determined to prevent any further Russian incursion into their territory.

“This would be no Crimea,” Sen. Joe Donnelly (D-Ind.) said at a news conference, adding that Putin would find himself having to explain why young Russian men were coming home in coffins. “Ukraine is ready to fight.”

Takeover of bases

Russia’s defense ministry announced Sunday it’s troops have taken control of 189 Ukrainian bases and facilities in Crimea. It is not clear how many, if any, still have a Ukrainian flag flying.

Russian troops are in control of the entire Ukrainian base at Feodosia, said a defense ministry official, and two senior commanders have been taken into custody.

Russian troops stormed the Marine base in the early morning hours, firing automatic weapons and using stun grenades, said Vladislav Seleznyov, a defense ministry spokesman. Some of the Marines had their hands bound, he said, and 80 to 100 have been gathered together in one spot.

Seleznyov said the Russians flew commander Dmitri Deliatizkii and his deputy, Rostilav Lomtev, off the base in a helicopter.

The commander of a Ukrainian base in Belbek, Yuli Mamchur, has been held by Russians since his base was stormed Saturday. Ukrainian television has reported it is believed he is being held in a jail in the port city of Sevastopol.

Early Monday morning, armed men with with face masks were in dinghies circling a Ukrainian ship that is boxed in at Donuslav bay, according to Seleznyov.

The base at Feodosia is home to an elite fighting force that is similar to the U.S. Marines, and it has a reputation for being highly trained and heavily armed.

Russian flags flew along the perimeter of the facility, while a few Ukrainian flags fluttered deep inside the base, sometimes alongside the Russian banner. As viewed from a neighboring soccer stadium, the front doors to two headquarters buildings were fortified with sandbags. On Sunday, Ukrainian marines could be observed loading long boxes into two trucks backed up to the entrance of one building.

Two Ukrainian officers on the base who came outside the gate to speak with reporters Sunday said the boxes contained weapons to be stored in an arsenal. They said their superiors are still hoping to negotiate permission to bring their weapons with them when they withdraw, a concession that the Russians did not grant to commanders at other Ukrainian bases.

“The important thing is, our flag is still up there,” said Capt. Aleksandr Lantukh, one of the officers, when asked about the plethora of Russian flags.

The two officers painted a picture of a base that is struggling to keep functioning as some troops have decided to return to Ukraine and others have decided to join the Russian armed forces in Crimea.

“Not only are the country and cities divided, our souls are divided,” Lt. Anatoly Mosgovy said. “We have friends. Some have families — the father is going to Ukraine and the mother and children will stay. But there is no tension in the base. We decided to be polite with each other and not exchange any bad words.”

Despite the determination to remain civil, Lantukh and Mosgovy made clear that they do not understand why some of their colleagues have decided to become Russian citizens and soldiers.

“A uniform is not for sale,” Lantukh said. “You cannot buy it. You cannot sell it.”

The Ukrainian soldiers’ departure is expected within days, through a safe corridor.

Citizens of Crimea voted overwhelmingly in a referendum March 16 to join Russia.

On Sunday, Putin ordered that police, civil defense, domestic intelligence and other governmental structures in Crimea must follow Russian law and procedures by March 29.

On Monday, the Russian ruble is to be introduced in Crimea as an official currency alongside the Ukrainian hryvnia.

The Ukrainian government in Kiev refuses to recognize the annexation.

DeYoung reported from Washington. Will Englund in Moscow, Kathy Lally in Kiev and Griff Witte in London contributed to this report.