US Secretary of State John Kerry is still waiting for Russian Foreign Minister Sergei Lavrov to return a telephone call to discuss North Korea's nuclear test, a top US official confirmed Thursday.

But, three days after Pyongyang's underground explosion triggered global outcry, State Department spokeswoman Victoria Nuland said the new top US diplomat was "relaxed" about the fact that Lavrov had not called back.

"The secretary would like to talk to him. It's up to him whether he wants to take that opportunity," Nuland said.

Lavrov has been traveling in Africa, and did issue a condemnation of North Korea's nuclear tests in the hours after it was carried out and called for "an adequate response" by the UN Security Council.

"It's not all that unusual in our recent experience that when Foreign Minister Lavrov is traveling he does not always engage in international phone calls on other subjects," Nuland added.

At one point last year, Kerry's predecessor Hillary Clinton also had trouble getting hold of Lavrov to discuss Syria.

Kerry was particularly keen to talk to Russia as it is a member of the six-party talks -- along with China, Japan, North Korea, South Korea, and the United States -- working to try to rein in Pyongyang's nuclear ambitions.

Washington and Moscow have testy relations and in recent months the Russian leadership has canceled a couple of joint operations with American agencies, as well as passing a law banning American families from adopting Russian children.

Kerry, who took office on February 1, has already spoken once to his Russian counterpart in what Nuland called "their introductory telephone call."