Russian President Vladimir Putin said on Thursday the latest US sanctions targeting two major oil firms were "serious", but not strong enough to significantly affect the economy.

The United States on Wednesday hit Russia's two biggest oil producers – Rosneft and Lukoil – with sanctions, the first against Russia since US President Donald returned to office.

"They (sanctions) are serious for us, of course, that is clear. And they will have certain consequences, but they will not significantly impact our economic well-being," Putin told reporters.

He added that the sanctions were an "unfriendly act" that "does not strengthen Russia-US relations, which have only just begun to recover."

Trump attempted to foster ties with Russia at the start of his term but has become increasingly frustrated with Putin's refusal to agree to a ceasefire, complaining that his peace talks with him were not going "anywhere".

The Republican president held off introducing new restrictions against Russia for months, but his patience snapped after plans for a fresh summit with Putin in Budapest collapsed.

Still, even after new sanctions and the summit's postponement, Putin appeared to remain open to dialogue.

"Dialogue is always better than confrontation or any disputes, and especially war. We have always supported the continuation of dialogue," Putin told journalists.

Asked about Putin's comment that the new sanctions would not have significant impact, Trump told reporters later on Thursday: "I'm glad he feels that way. That's good. I'll let you know about it in six months from now."

With Ukraine asking US and European allies for long-range missiles to help turn the tide in the war, Putin also warned that Moscow's response to strikes deep into Russia would be "very serious, if not overwhelming".

Separately, EU and Nato member Lithuania on Thursday said two Russian military aircraft briefly entered its airspace, prompting a formal protest and a reaction from NATO forces, while Russia denied the incident. (Agencies)