WASHINGTON (Reuters) - A defense pact between Russia and North Korea signed by their two leaders is a cause of concern for the United States but is no surprise, the White House said on Thursday.

White House national security spokesperson John Kirby told reporters the agreement is a sign of Russia's desperation as it needs foreign assistance in its war against Ukraine.

Russian President Vladimir Putin said on Thursday after the announcement of the defense pact the previous day that Russia might supply weapons to North Korea in what he suggested would be a mirror response to the Western arming of Ukraine.

Putin also said U.S. ally South Korea would be making "a big mistake" if it decided to supply arms to Ukraine, and that Moscow would respond to such a move in a way that would be painful for Seoul.

Speaking ahead of Putin's remarks, the White House's Kirby said the defense pact was "no surprise."

"We've been talking about this and warning about a burgeoning defense relationship between these two countries now for many months through a series of downgraded intelligence that we've put out there," he said.

"We've been sharing information publicly about how the DPRK has been enabling Russia's war in Ukraine and about their weapons transfers. Obviously it's something we've taken seriously."

The United States and Ukraine say North Korea, which Kirby referred to by the initials of its official name, has already provided Russia with significant quantities of artillery shells and ballistic missiles, which Moscow and Pyongyang deny.

Kirby said the Biden administration had been bolstering its alliances and partnerships in the Indo-Pacific throughout its three-and-a-half years in office, citing a trilateral deal with Japan and South Korea, the AUKUS pact to supply Australia with nuclear-powered submarines and stepped-up ties with the Philippines.

(Reporting by Steve Holland, Doina Chiacu and David Brunnstrom; writing by David Brunnstrom; Editing by Andrew Heavens)

By Steve Holland