* S.Korea to request high-level talks with N.Korea for late March
* U.S., S.Korea may hold summit between N.Korea summits -South official
* Report shows activity at North Korean reactor
SEOUL/WASHINGTON, March 16 (Reuters) - South Korea said on Friday it was seeking high-level talks this month with North Korea to prepare for a summit and that the South Korean president may meet Donald Trump before the U.S. president's planned meeting with the North Korean leader.
The proposed North-South talks in late March would cover key agenda topics and other details of a pending summit between South Korean President Moon Jae-in and North Korean leader Kim Jong Un, Moon's chief of staff, Im Jong-seok, said.
If North Korea agrees to the talks, they would offer an opportunity for Pyongyang to break its silence on what Seoul says is Kim's desire to meet Trump and Moon and his willingness to freeze his country's nuclear and missile programs.
"We've decided to narrow down the agenda topics to denuclearising the Korean peninsula, securing permanent peace to ease military tension and new, bold ways to take inter-Korean relations forward," Im, the head of South Korea's summit preparation team, told reporters.
Im said Moon may meet Trump after an inter-Korean summit but before Trump's planned summit with Kim in May.
Trump and Moon spoke by telephone on Friday and Moon pledged to cooperate closely with Washington on summit diplomacy, South Korea's Yonhap news agency reported. Trump asked South Korean officials to show flexibility in trade negotiations with the United States in the call, the South Korean presidency said.
Even amid North Korea tensions, Trump has repeatedly denounced a U.S. free trade deal with ally South Korea as "unfair" and threatened to scrap it altogether on multiple occasions.
The White House confirmed the call took place but gave no immediate details.
Senior South Korean officials met Kim Jong Un in Pyongyang this month and told Washington that the North Korean leader was open to giving up his nuclear weapons if North Korea's security was guaranteed.
Trump responded with a surprise announcement that he was willing to meet Kim in a bid to resolve the crisis over North Korea's development of nuclear missiles capable of hitting the United States.
TALKS IN SWEDEN
North Korea's state media has yet to comment on the content of Kim's meeting with the South Koreans, but North Korean Foreign Minister Ri Yong Ho arrived in Sweden this week for talks with his Swedish counterpart Margot Wallstrom, prompting speculation that he could lay the groundwork for a Trump-Kim summit.
Swedish Prime Minister Stefan Lofven said on Friday that Sweden was ready to act as a facilitator to help resolve tensions over North Korea. Sweden's embassy represents U.S. interests in Pyongyang.
"If we can act as a facilitator such that something comes out of this, then we will of course do so," he told a joint news conference with German Chancellor Angela Merkel in Berlin.
South Korea's Foreign Minister Kang Kyung-wha was in Washington on Friday for talks expected to focus on North Korea and trade, as was Japanese Foreign Minister Taro Kono, who met U.S. Deputy Secretary of State John Sullivan in the morning.
Kang was expected to hold talks later on Friday with Sullivan, who is standing in after Trump fired Rex Tillerson as U.S. secretary of state this week. It was not known whether the two Asian ministers would meet CIA Director Mike Pompeo, who Trump has tapped to replace Tillerson.
The Japanese and South Korean ministers were expected to meet in Washington on Saturday.
In a phone call with South Korean President Moon on Friday, Japanese Prime Minister Shinzo Abe expressed a wish for talks with North Korea following Pyongyang's planned summits with South Korea and the United States, South Korea's presidential spokesman, Kim Eui-kyeom said.
On Thursday, the head of the U.S. Pacific Command, Admiral Harry Harris, cautioned that the United States could not be overly optimistic about the outcome of any summit between Trump and Kim and must go into it with "eyes wide open".
Harris said he believed the United States would stick to its demand for the "complete, verifiable, irreversible denuclearisation" of the Korean peninsula.
A report on Friday by intelligence analysts at Jane's by IHS Markit said satellite imagery from Feb. 25 showed gas emissions from a stack at the North's experimental light water reactor, suggesting preliminary testing had likely begun.
The reactor could be used to produce weapons-grade plutonium, but North Korea is believed to already have enough fissile material for multiple nuclear bombs, according to Joshua Pollack, a senior research associate at the Middlebury Institute of International Studies at Monterey.
An official at South Korea's defence ministry said authorities were aware of the Jane's report, which follows a similar one on the 38 North website earlier this month that said a nearby reactor had also continued to show signs of operation.
The U.S. State Department declined to comment on the Jane's report, calling it an intelligence issue. (Reporting by Christine Kim and Josh Smith in Seoul and Lesley Wroughton, David Brunnstrom and Steve Holland in Washington; Writing by David Brunnstrom Editing by Mary Milliken and James Dalgleish)