North Korea has not responded to multiple behind-the-scenes communications from U.S. President Joe Biden’s administration since mid-February, including messages sent to North Korea’s mission to the United Nations, NK News has confirmed.

The silence comes as the Biden administration continues to work on its North Korea policy review, which will determine how Washington approaches denuclearization and peace on the peninsula after four years of former President Donald Trump’s unconventional, summit-driven approach. So far, North Korea’s lack of response to Washington’s backdoor diplomatic efforts has some analysts concerned.