SEOUL, South Korea — North Korea appears to have resumed work at its nuclear test site after a perplexing series of volleyball matches were held there, according to analysts who studied satellite images of the site, renewing concerns that a major weapons test could be imminent.

Many observers had feared that North Korea would test a nuclear device at the site around April 15, the birthday of Kim Il-sung, the North’s founding president and the grandfather of the current leader, Kim Jong-un. But Mr. Kim’s government celebrated the day instead with a military parade in Pyongyang, the capital, during which a fleet of missiles were rolled out, including what analysts believed were never-before-seen long-range ballistic missiles.

North Korea carried out a missile test on Sunday, but it was considered an embarrassing failure, with the projectile exploding immediately after liftoff.

But North Korea is preparing to celebrate another major holiday this coming week: Tuesday will be the 85th anniversary of the founding of the Korean People’s Army, and the North often uses such occasions to show off its military advances.