The United States has urged Prime Minister Shinzo Abe to refrain from visiting North Korea, with Secretary of State John Kerry warning such a trip could disturb trilateral coordination involving Tokyo, Washington and Seoul to rein in Pyongyang’s missile and nuclear programs, sources familiar with Japan-U.S. relations said Tuesday.

In a telephone conversation with Foreign Minister Fumio Kishida on July 7, Kerry requested that Japan hold behind-the-scenes consultations with the United States in advance should Tokyo consider a visit to North Korea by Abe, according to the sources.

The top U.S. diplomat also expressed displeasure over Japan’s policy of gradually lifting its unilateral sanctions on North Korea depending on progress in the new round of investigations into the fate of Japanese nationals abducted by the North in the 1970s and 1980s.

Kerry’s remarks, which were not made public following the 40-minute conversation with Kishida, indicated for the first time that the Obama administration is wary about recent developments in Tokyo-Pyongyang ties.

One of the sources quoted Kerry as saying it would not be good to see Japan move forward alone from trilateral effort, and should Abe visit North Korea, the United States wants Japan to hold sufficient consultations in advance, rather than just informing Washington at short notice.

Kishida said during a June 3 session of the Upper House Foreign Affairs and Defense Committee that a visit by Abe may be one way to settle abduction issue.

Kerry, who spent most of the telephone conversation talking about the sanctions on North Korea, asked Japan to be careful about lifting any more, according to the sources.

On July 4, Pyongyang declared the launch of a new organization aimed at looking into the whereabouts of missing Japanese nationals suspected of having been abducted to North Korea. In return, Tokyo lifted some of its sanctions.

Kishida was quoted as telling Kerry that his remarks in the Diet were inconsequential and that the administration is “not considering a visit to North Korea by the prime minister at all.”

He also reportedly said Japan is not considering lifting any more sanctions.

Despite removing some of the unilateral sanctions, such as travel restrictions between the two countries and a ban on the entry of North Korean-registered ships into Japanese ports provided they are arriving for humanitarian purposes, Japan has kept intact U.N.-mandated sanctions on the North for its nuclear tests and missile launches.

Japan officially lists 17 nationals as abductees, but North Korea is suspected of involvement in many more disappearances. While five of the 17 were repatriated in 2002, Pyongyang claims eight have died and four others never entered the country.

The number of missing Japanese who may have been abducted by North Korea ranges from about 470 as listed by the Investigation Commission on Missing Japanese Probably Related to North Korea, a Tokyo-based citizens’ group, to 860 as estimated by the National Police Agency.

The abduction issue has prevented the two countries from normalizing diplomatic relations.