Nearly 15,000 South Korean and U.S. troops kicked off a 12-day amphibious landing drill Thursday, the largest for two decades, a day after North Korea launched two mid-range ballistic missiles.

Code-named Ssang Yong ("Twin Dragons"), the exercise on the South's southeastern coast will last until April 7 and involve around 10,000 U.S. troops.

North Korea issued a scathing personal attack Thursday on South Korean President Park Geun-Hye, accusing her of breaking a moratorium on cross-border insults and behaving like a "blabbering" peasant woman.

The attack referenced a speech Park made Monday at a nuclear summit in The Hague in which she voiced concern that Pyongyang's nuclear material could end up in terrorist hands, and warned of a possible Chernobyl-style disaster at the North's main Yongbyong atomic complex.

A senior U.S. lawmaker vowed Wednesday to seek stiffer sanctions against North Korea due to human rights concerns after a stinging U.N. report likened the regime's abuses to the Holocaust.

Representative Ed Royce, chairman of the House Foreign Affairs Committee, told a hearing that he would seek to enact a bill that would broaden already sweeping sanctions by punishing foreign governments that assist Kim Jong-Un's regime.

Chancellor Angela Merkel pledged Germany's support Wednesday during a visit by South Korea's president for efforts to unify the Korean peninsular, saying its own reunification gave it a "duty" to help others.

"We would like very much to support Korea in this important issue," Merkel told a joint press conference with President Park Geun Hye, who is on a state visit to Germany.

The United States has said that North Korea's test firing of two medium-range missiles into the Sea of Japan represents "a troubling and provocative escalation" on the Korean Peninsula.

America is closely coordinating with allies and partners, including in the U.N. Security Council, "to take the appropriate measures in response" to Pyongyang's latest "violation of U.N. Security Council resolutions," the State Department said.

North Korea on Monday fired a fresh warning that it would take nuclear "measures" if the United States does not end what the communist state described as provocations.

North Korea's ambassador to the United Nations, Ri Tong-Li, said that Pyongyang was "ready" with measures that would "demonstrate the power of the self-defensive nuclear deterrent."

North Korea test-fired 16 short-range rockets into the sea on Sunday, the South's military said -- the latest of dozens launched in recent weeks in apparent protest at joint drills between Seoul and Washington.

The launches took place between about 1:00 am (1600 GMT Saturday) and 2:30 am off the North's east coast, a South Korean defense ministry spokesman told AFP.

North Korea test-fired 30 short-range missiles into the sea on Saturday, South Korea's Joint Chiefs of Staff said, the latest in a series of launches despite calls from Seoul and Washington to stop "provocative actions".

"North Korea fired off 30 short-range missiles between 4:00 am and 6:10 am (1900-2110 GMT Friday) this morning from its east coast into the Sea of Japan (East Sea)," said a spokesman for the Joint Chiefs.

Recent satellite images of North Korea's nuclear and rocket test sites do not point to any imminent tests, a U.S. think-tank said Friday, despite last week's threat by the communist state to flex its nuclear muscle.

The images indicate that excavation continues on a new tunnel at the remote Punggye-ri test site in the northeast, but there is little or no activity at other key parts of the facility, said the closely-followed 38 North website of the Johns Hopkins University's U.S.-Korea Institute.

Japan and North Korea are moving to restart formal government-level talks, a foreign ministry official said Thursday, after a shift over the contentious issue of Pyongyang's past abductions of Japanese citizens.

The step forward came as diplomats held informal talks on the sidelines of a two-day humanitarian meeting in the Chinese city of Shenyang between Red Cross officials from the two countries, the Japanese official said.