Satellite photos show more than 600 vehicles mobilized at a training ground in North Korea’s capital in a sign that the country is preparing for a large-scale military parade. 

Citing imagery taken by Planet Labs, Radio Free Asia said Wednesday that the North on Monday mobilized around 600 to 650 vehicles at the Mirim airport in Pyongyang -– where rehearsals for previous military parades have taken place –- signaling a forthcoming gathering. 

There were about 150 vehicles in February, but the number increased four-fold this month, it said. These vehicles appear to carry personnel and equipment for the parade, and the number is expected to continue to increase as the parade nears. 

The North is estimated to have gathered more than 7,500 troops for the parade, according to the RFA. The South Korean military official estimated early this week that the number was more than 6,000 troops. 

Vehicle and troop mobilization at the Mirim airport comes amid speculation that Pyongyang may hold a major military parade in time for a key political anniversary on April 15 -- the 110th birthday of state founder Kim Il-sung, the later grandfather of leader Kim Jong-un, which is known as the Day of the Sun in the North. Pyongyang tends to celebrate every fifth and 10th anniversary with larger events, including military parades.

Concerns grow as the North may use the parade to show off new weapons systems, including a long-range ballistic missile, in a show of force to other countries and to bolster domestic unity. Since the start of this year, the North has been ramping up tensions by launching a barrage of missiles, including a new intercontinental ballistic missile system, an intermediate-range ballistic missile and a purported hypersonic missile. 

The South Korean government is closely watching the North’s moves in regards to a potential military parade in cooperation with Washington, according to an official here. 

“We view the possibility of a military parade as high, considering the North’s precedent and announcement as well as signs detected by our military,” an official at the Unification Ministry said Thursday. 

The official said that it remain to be seen exactly when the North would stage the parade, but added that in the past, the North held a military parade for Kim Il-sung’s 100th and 105th birth anniversaries. 

On Thursday, the North’s official news outlet, the Korean Central News Agency, said the country plans to hold various large-scale celebratory events in time for the Day of the Sun.