SEOUL, South Korea — North Korea failed to launch two intermediate-range ballistic missiles on Thursday, the South Korean Defense Ministry said, adding to a string of unsuccessful weapons tests in the past two weeks.

In the first attempted launch, the powerful Musudan missile crashed into the sea seconds after ignition. Hours after that attempt, North Korea fired another missile of the same type, but that test also failed, the ministry said.

Thursday’s attempts bring the total failures in launching the Musudan — one of the North’s most powerful missiles deployed or under development — to three in the past two weeks, according to South Korean officials.

Image The leader of North Korea, Kim Jong-un, in October 2015. Mr. Kim ordered his military to conduct more tests of ballistic missiles capable of delivering nuclear warheads. Credit... Wong Maye-E/Associated Press

The successive failures are a potential embarrassment for North Korea’s leader, Kim Jong-un, just days before a rare congress of his ruling Workers’ Party, the North’s biggest political meeting in decades.