WASHINGTON –

The United States accused Russia on Thursday of actively fighting in eastern Ukraine in support of separatist forces and said it was considering a range of possible responses, including increased sanctions.

"It is clear that Russia has not only 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," U.S. State Department spokeswoman Jen Psaki told a media briefing.

In response, she said, "we have a range of tools at our disposal," adding that increased sanctions on Russia were "the most effective tool, the best tool."

Psaki said the United States has seen a "pattern of escalating aggression" by Russia in Ukraine.

Ukraine's president said earlier on Thursday that Russian troops had entered his country in support of pro-Moscow rebels who captured a key coastal town, sharply escalating the five-month-old separatist war in the East European country.

Psaki said the United States was focused on providing nonlethal assistance to the Kiev government.

"We have provided a range of nonlethal assistance, as has a number of other countries in Europe. We have not ruled out options," she said.

"A military solution is not what we think is the appropriate approach so we're taking every tool that we can to see if we can reach a solution here through diplomatic means," Psaki said.

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

The sanctions have drastically limited access to Western banking money and modern oil technology, which is hurting Russia's economy as it relies on energy for half of its budget revenue.