Two US journalists are reported to have gone on trial in North Korea, on charges of committing "hostile acts".
Euna Lee, a Korean-American, and Laura Ling, a Chinese-American, were arrested on 17 March after allegedly crossing into North Korea from China.
The US has dismissed the charges as "baseless" and said the two women should be released immediately.
The trial comes amid growing tensions in the region following North Korea's recent nuclear and missile tests.
In a new sign of friction, a North Korean patrol boat is reported to have briefly entered South Korean waters.
The South's Yonhap news agency said the vessel crossed the disputed maritime border on Thursday, but retreated when issued with a verbal warning.
Meanwhile US Deputy Secretary of State James Steinberg has said China's relationship with North Korea seems to have cooled recently, since Pyongyang's latest nuclear test.
During a meeting with the South Korean president in Seoul, Mr Steinberg said Pyongyang was failing to read changes in Beijing's position.
"It would be a mistake for the North to believe that it can obtain what it wants through negotiations after staging provocative acts," he said.
Appeal for clemency
Euna Lee and Laura Ling were arrested by North Korean guards while working on the China-North Korea border on a story about refugees for California-based internet broadcaster Current TV.
Some reports have suggested that the women were arrested while on Chinese soil, but Pyongyang's state media says they had illegally entered North Korea.
North Korea said the trial would begin at 0600 GMT and the Russian news agency Itar-Tass reports that the hearing has now begun, although there has been no confirmation from Pyongyang.
If the women are found guilty, analysts say they face at least five years detention with hard labour in a North Korean prison camp.
The women's families have appealed for clemency and asked North Korea and the United States not to link the case to the current diplomatic standoff between them.
But some analysts have suggested that a guilty verdict is almost certain.
Amnesty International has said it is highly unlikely the two women will face a fair trial because of the judicial system's lack of independence or transparency.