U.S. President Barack Obama answers questions in the White House Press Briefing Room ahead of a meeting with his national security council in Washington, August 28, 2014. REUTERS/Larry Downing

By Roberta Rampton and Steve Holland

WASHINGTON (Reuters) - The United States openly accused Russia on Thursday of sending combat forces into Ukraine and threatened to tighten economic sanctions, but Washington stopped short of calling Moscow's intensified support for separatist forces an invasion.

President Barack Obama again ruled out U.S. military action, saying there must be a diplomatic solution. But he said he would reaffirm U.S. commitment to NATO allies in the region at a summit next week.

"We are not taking military action to solve the Ukrainian problem. What we're doing is to mobilize the international community to apply pressure on Russia," he told reporters at the White House.

After days in which pictures appeared of Russian soldiers in uniform and Russian weapons in action in a renewed offensive against Ukrainian troops, Washington brushed aside Moscow's disavowals of direct involvement in the fighting.

"Russia has ... stepped up its presence in eastern Ukraine and intervened directly with combat forces, armored vehicles, artillery, and surface-to-air systems, and is actively fighting Ukrainian forces as well as playing a direct supporting role to the separatists' proxies and mercenaries," State Department spokeswoman Jen Psaki told a media briefing.

"We have a range of tools at our disposal" to respond, she said, and added that increased sanctions on Russia were "the most effective tool, the best tool."

At an emergency meeting of the U.N. Security Council, U.S. Ambassador Samantha Power poured scorn on Moscow's declarations that its forces were not involved in Ukraine. "It has manipulated. It has obfuscated. It has outright lied," she said.

"The mask is coming off. In these acts, these recent acts, we see Russia’s actions for what they are: a deliberate effort to support, and now fight alongside, illegal separatists in another sovereign country."

British U.N. Ambassador Mark Lyall Grant told the meeting:

"Formed units of the armed forces of the Russian federation are now directly engaged in fighting inside Ukraine against the armed forces of Ukraine. These units consist of well over 1,000 regular Russian troops equipped with armored vehicles, artillery and air defense systems."

RUSSIAN RESPONSE

Russia's U.N. ambassador, Vitaly Churkin, responded: "There are Russian volunteers in eastern parts of Ukraine. No one is hiding that." Moscow has said some Russians have, in their own time, gone to Ukraine to support the cause of the separatists.

Churkin said he had a message for the United States: "Stop interfering in the internal affairs of sovereign states."

Obama, who will visit Russia's tiny neighbor Estonia before attending a NATO summit in Wales next Thursday and Friday, said Russian President Vladimir Putin had ignored opportunities to find a diplomatic solution.

In Estonia, he said he would "reaffirm our unwavering commitment to the defense of our NATO allies."

The White House said Ukrainian President Petro Poroshenko would meet with Obama at the White House on Sept. 18 in a visit that "will highlight the United States’ firm commitment to stand with Ukraine as it pursues democracy, independence, and stability."

Obama spoke by phone on Thursday with German Chancellor Angela Merkel, who has played a big role trying to resolve the crisis. The White House said the two agreed the United States and Europe should consider more sanctions.

The United States and European countries have imposed increasingly stiff sanctions on Russia over its annexation of Crimea in March and its role in subsequent fighting in Ukraine.

Asked directly if he would refer to the Russian intervention as an invasion, Obama said: "I consider the actions that we've seen in the last week a continuation of what's been taking place for months now."

He said: "The separatists are backed, trained, armed, financed by Russia. Russia determined that it had to be a little more overt in what it had already been doing, but it's not really a shift."

(Reporting by Roberta Rampton, Steve Holland, Mark Felsenthal and Jim Loney in Washington and Lou Charbonneau in New York; Writing by David Storey; Editing by David Gregorio and Ken Wills)