By Michelle Nichols

UNITED NATIONS (Reuters) – As the United States pressures China to enforce United Nations sanctions on its ally North Korea, Washington is concerned that Russia could provide support to Pyongyang and fill any vacuum left by Beijing, U.S. Ambassador to the United Nations Nikki Haley said on Tuesday.

“I’m concerned that Russia may backfill North Korea,” Haley told U.S. lawmakers in Washington. “We don’t have proof of that, but we are watching that carefully.”

While Washington has urged countries to downgrade ties with Pyongyang over its nuclear and ballistic missile programs, a cross-border ferry service was launched in May between North Korea and neighboring Russia. Russian President Vladimir Putin has said the world should talk to, rather than threaten, North Korea.

“We just need to keep the pressure on China, we need to keep our eyes on Russia, and we need to continue to let the North Korea regime know we are not looking for regime change … we just want them to stop the nuclear activity,” Haley said.

The U.N. Security Council first imposed sanctions on North Korea in 2006 over its ballistic missile and nuclear programs and has ratcheted up the measures in response to five nuclear tests and two long-range missile launches. The government in Pyongyang is threatening a sixth nuclear test.

The Trump administration has been pressing China aggressively to rein in its reclusive neighbor, warning that all options are on the table if Pyongyang persists with its nuclear and missile development programs.

Beijing has repeatedly said its influence on North Korea is limited and that it is doing all it can, but U.S. President Donald Trump last week said China’s efforts had failed.

The United States has struggled to slow North Korea’s programs, which have become a security priority given Pyongyang’s vow to develop a nuclear-tipped missile capable of hitting the U.S. mainland.

“The pressure on China can’t stop,” Haley said. “We have to have China doing what they’re supposed to. At the same time all other countries need to make sure they’re enforcing the sanctions that the Security Council has already put in place.”

Trump, increasingly frustrated with China over its inaction on North Korea and bilateral trade issues, is now considering possible trade actions against Beijing, senior administration officials told Reuters.

The United States also plans to place China on its global list of worst offenders in human trafficking and forced labor, sources said, a step that could aggravate tensions with Beijing.

(Additional reporting by Patricia Zengerle, editing by G Crosse)