UPDATED Jan. 8, 2015, 10:45 p.m. ET with South Korea resuming propaganda blasts.

U.S. intelligence-gathering aircraft have been taking off from an American air base in southern Japan, a day after North Korea said it tested a hydrogen bomb.

At least three planes departed Thursday from Kadena Air Base on the Japanese island of Okinawa.

It wasn't clear what they were doing, but they could be involved in trying to determine what kind of nuclear device North Korea detonated.

One of the aircraft seen departing, the RC-135S, collects optical and electronic data from ballistic targets. It is used for arms treaty compliance verification and U.S. strategic and missile defense development.

White House spokesman Josh Earnest told reporters in Washington on Wednesday that "the United States will be collecting additional evidence" and working closely with other countries in the region to learn more about the North Korean test.

There have been widespread doubts that the bomb was actually a hydrogen bomb, and experts from different countries are working to determine what nuclear technology the North has.

Propaganda blasts resume

Part of a panel of loudspeakers installed at the demilitarised zone (DMZ) between South and North Korea that the South uses to play propaganda messages.

On Friday, South Korea resumed the cross-border propaganda broadcasts that Pyongyang considers an act of war.

The South's presidential office said on Thursday it would resume broadcasts in retaliation for the bomb tests. South Korea's Yonhap news agency said that loudspeakers pointed at the border started blaring messages at noon, and that frontline troops near the speakers were on highest alert.

Yonhap said Seoul had deployed missiles, artillery and other weapons systems near the border to swiftly deal with any possible North Korean provocation.

There was no immediate reaction by the North, but its response could be especially harsh because of the high emotions surrounding the likely birthday of Kim, who is believed to be in his early 30s. North Korean military forces often compete to show their loyalty to the leader. The North's state media has yet to mention Kim's birthday or South Korea's loudspeaker campaign.

Seoul used to blast messages across the border through the 11 giant speakers. It had briefly resumed broadcasting in August after an 11-year break, but stopped after the two Koreas exchanged artillery fire, and it managed to strike an agreement with Pyongyang on a package of measures aimed at easing animosities.

The loudspeakers have also been known to blast Kpop music, world news and weather forecasts as well as criticism of the North's nuclear test, its troubled economy and dire human rights conditions, according to Seoul's Defense Ministry.

Additional reporting by Mashable.