U.S. State Department spokeswoman Jen Psaki said during her regular Monday press conference in Washington that North Korea has "on numerous occasions" committed to abandoning its nuclear weapons program.
"We'll continue to hold them to their commitment and to their international obligations," she said. "And beyond that, the ball is in their court to take those relevant steps."
Chinese Foreign Ministry spokeswoman Hua Chunying said recently Beijing wanted its negotiating partners to work on building the trust necessary to resume nuclear negotiations. Six-party talks between the two Koreas, China, the United States, Russia and Japan stalled in 2009.
Experts on North Korea at Johns Hopkins University wrote Monday on their blog, 38 North, that North Korea seems to be conducting new work at a satellite launch station.
"Evidence is growing that these activities are intended to support the two main priorities for North Korea's rocket program -- launches of larger rockets and of new mobile missiles," they said.
Tensions escalated in the region when North Korea conducted a nuclear test in February, its third since 2006. Each of the nuclear tests coincided with rocket launches.