By Joyce Lee and Jeff Mason

SEOUL/WASHINGTON (Reuters) - North Korean leader Kim Jong Un told Russian President Vladimir Putin that peace and security on the Korean peninsula depended on the United States, warning that a state of hostility could easily return, North Korean media said on Friday.

Kim's remarks during talks with Putin on Thursday appeared aimed at pushing Washington to be more flexible on North Korean demands for an easing of international sanctions.

U.S. President Donald Trump, who ended a second summit with Kim in February without a deal for North Korea to give up its nuclear weapons, seemed unperturbed, saying a lot of progress was being made toward an agreement and welcoming Putin's support.

Putin said after holding his first face-to-face talks with Kim in the Russian Pacific port of Vladivostok on Thursday that U.S. security guarantees would probably not be enough to persuade Pyongyang to give up its nuclear program.

He said he believed any U.S. guarantees might need to be supported by the other nations involved in previous six-way talks on the nuclear issue, which included Russia, China, Japan and South Korea as well as the United States and North Korea.

"I think we're doing very well with North Korea. A lot of progress is being made," Trump told reporters at the White House. "I appreciated President Putin's statement yesterday. He wants to see it done also. I think there's a lot of excitement for getting a deal done with North Korea."

The Trump-Kim summit in Hanoi in February collapsed without progress on a U.S. demand that North Korea give up its nuclear weapons and North Korean demands for easing of sanctions.

Kim has said he would wait until the end of the year for the United States to be more flexible.

"The situation on the Korean peninsula and the region is now at a standstill and has reached a critical point where it may return to its original state as the U.S. took a unilateral attitude in bad faith at the recent second DPRK-U.S. summit talks," North Korea's official KCNA news agency quoted Kim as saying.

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The Democratic People's Republic of Korea (DPRK) is North Korea's official name.

"The DPRK will gird itself for every possible situation." KCNA quoted Kim as saying.

William Hagerty, the U.S. ambassador to Japan, told a Washington think-tank on Thursday that Kim’s contacts with both Russia and China were part of an effort to seek relief from sanctions and it was important the international community continue to enforce these.

"The fact you see Kim Jong Un meeting with Vladimir Putin underscores the fact that the sanctions are working and the sanctions are putting extreme economic pressure on the North Korean regime," Hagerty said.

"What we see is an outreach to try to find a way to deal with it. There is a much simpler way to deal with it and that is to denuclearize," he said.

SECURITY GUARANTEES

The first face-to-face talks between Putin and Kim, held on an island off the Russian Pacific city, did not appear to yield any major breakthrough.

The two discussed ways to promote strategic communication and tactical collaboration in the course of ensuring peace and security on the Korean peninsula and beyond, KCNA said.

Putin said a deal on North Korea's nuclear program was possible by moving step by step to build trust.

Russia was for years a participant in the six-party talks, which have not been held since 2009.

"They only need guarantees about their security. That's it. All of us together need to think about this," Putin told reporters after talks with Kim. Such guarantees would have to be international, legally binding, and vouch for North Korea's sovereignty, Putin said.

Russia and North Korea agreed to increase cooperation and Putin accepted an invitation by Kim to visit North Korea, KCNA said. No date was announced.

"North Korea seems to be trying to expand its negotiating position with the U.S.," said South Korea's ambassador to the United States, Cho Yoon-je, according to the Yonhap news agency.

"The U.S. continues to send a message to North Korea through channels at every level that it is open to dialogue ... The expectation seems to be that the North may respond once Chairman Kim Jong Un's diplomatic schedule is completed."

(Reporting by Joyce Lee; Additional reporting by David Brunnstrom and Jeff Mason in WASHINGTON, Hyonhee Shin in SEOUL and Maria Vasilyeva in VLADIVOSTOK; Editing by Jack Kim, Robert Birsel and James Dalgleish)