The tests, which included launching an intercontinental ballistic missile last month that could conceivably strike deep inside the United States, are "clear violations of multiple UN Security Council resolutions," said Sung Kim, the US Special Representative for the Democratic People's Republic of Korea.

"We must work together to come up with a decisive response to DPRK's provocative behavior," Kim said, calling the launches "escalatory behavior."

Kim was speaking as he met in Washington with South Korea's Special Representative for Korean Peninsula Peace and Security Affairs, Noh Kyu-duk.

On March 24 Pyongyang launched its most powerful missile yet, an ICBM that has the potential of delivering a nuclear weapon to the US east coast.

The test launch was one of a series over the past year and underscored North Korea's refusal to engage in talks with other countries over limiting its nuclear threat, despite the heavy economic sanctions on the country.

Washington has meanwhile failed to persuade China and Russia to support intensified UN sanctions on North Korea.

"We remain open to diplomacy, but it really is up to Pyongyang to decide the path forward," Kim said.