North Korea tested a rocket engine earlier this year in a potential bid to further develop its missile capability, a U.S. think-tank said Thursday after reviewing new satellite images.

Images of the Tongchang-ri rocket base in the country's northwest indicated the North conducted at least "one or more rocket engine tests" in late March or early April, the U.S.-Korea Institute at Johns Hopkins University said.

North and South Korea agreed in principle Wednesday to hold talks on restarting a family reunion program, as separate discussions on reopening a jointly-run industrial estate failed to make progress.

The North also proposed official talks on resuming suspended cross-border tours, in a message sent through the border truce village of Panmunjom, the South's unification ministry said.

South Korea said talks with North Korea on reopening a jointly-run industrial estate ended without agreement Wednesday, but the two sides agreed to meet again next week.

The South's chief delegate Suh Ho said talks on restarting the Kaesong industrial complex's mothballed factories would continue on July 15, after about four hours of meetings that started Wednesday morning.

North and South Korea on Sunday made a crucial step forward in winding down months of high tension, agreeing to reopen a joint industrial zone seen as the last remaining symbol of cross-border reconciliation.

The deal follows months of friction and threats of war by Pyongyang after its February nuclear test attracted tougher U.N. sanctions, further squeezing its struggling economy.

North and South Korea held rare talks Saturday on re-opening a joint industrial zone seen as the last remaining symbol of cross-border reconciliation.

The talks -- delayed by nearly two hours -- follow months of friction and threats of war by Pyongyang after its February nuclear test attracted tougher U.N. sanctions, further squeezing its struggling economy.

Russian diplomats on Thursday held several hours of talks with a high-ranking North Korean official, urging Pyongyang to ramp up efforts to revive the stalled six-party nuclear negotiations.

"The Russian side stressed the need to unite efforts in the interests of easing tensions (and) creating conditions for the earliest resumption of the six-party talks," the foreign ministry said in a statement after top Russian diplomats met with North Korea's first vice foreign minister Kim Kye-Gwan.

South Korea on Thursday proposed fresh talks with North Korea aimed at re-opening a shuttered joint industrial zone at Kaesong, a government spokesman said.

The Unification Ministry sent a message to the North Korean government proposing that three officials from each side meet at the border truce village of Panmunjom on Saturday, Spokesman Kim Hyung-Seok said.

Dozens of South Korean firms on Wednesday threatened to withdraw from a shuttered joint industrial zone in North Korea, complaining they fell victim to political bickering between the two Koreas.

Representatives of the 123 South Korean companies with factories in Kaesong have repeatedly urged North and South Korea to open talks to revive the moribund industrial park.

A high-ranking North Korean official with long experience in nuclear talks left Tuesday for Russia, state media said, as the communist state steps up a diplomatic offensive after months of tension.

First vice foreign minister Kim Kye-Gwan was leading a delegation en route to the Russian capital, the official Korean Central News Agency said in a brief report.

U.S. Secretary of State John Kerry sought Monday to present a united front against North Korea's nuclear ambitions as he met his counterparts from China, Japan and South Korea.

Kerry, meeting Chinese Foreign Minister Wang Yi on the sidelines of an Asia-Pacific meeting in Brunei, said that the two "both reaffirmed strongly the seriousness" of commitment to the denuclearization of North Korea.