SEOUL (Reuters) - U.S. President-elect Donald Trump’s national security adviser says North Korea’s nuclear program would be given a high priority under the new administration, a South Korean official who held talks with him said on Saturday. Michael Flynn, one of Trump’s closest advisers, also said he would work to strengthen the U.S. alliance with South Korea, calling the relationship “vital,” the South’s deputy presidential national security adviser Cho Tae-yong was quoted as saying by Yonhap news agency. Cho was leading a South Korean delegation to the United States to meet with key advisers to the president-elect to discuss the two countries’ response to the North’s pursuit of nuclear weapons in defiance of international sanctions. Cho spoke to South Korean reporters in Washington following the meeting with Flynn, Yonhap said. Flynn is a retired Army lieutenant general and a military intelligence veteran of three decades who has championed Trump’s promises to take a more aggressive approach to terrorism. His appointment as national security adviser this week does not require Senate confirmation.     The North conducted its fourth and fifth nuclear tests this year under young leader Kim Jong Un, who has vowed to build a nuclear arsenal and ballistic missiles to deliver them. The U.N. Security Council has held discussions to adopt a toughened new sanctions resolution following the North’s Sept. 9 nuclear blast.  U.S. President Barack Obama has been criticized by Congressional Republicans that his policy of “strategic patience” was a failure and that he must make full use of sanctions authorities given to him by Congress.  Trump pledged his commitment to defend South Korea under an existing security alliance during a phone call with South Korean President Park Geun-hye, Yonhap said last week. Trump had suggested during the election campaign he would be willing to withdraw U.S. military stationed in South Korea unless Seoul paid a greater share of the cost of the deployment. There are about 28,500 U.S. troops based in South Korea in combined defense against North Korea. 