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Hong Kong (CNN)North Korea may be seeking to outpace its southern neighbor by testing multiple missiles as Seoul prepares to deploy a controversial US missile defense system.

In what has become almost routine, on Monday North Korea fired three ballistic missiles from a base in the west across the country into the Sea of Japan.

The missiles all fell within 250 kilometers of Japan's Okushiri Island, the country's Defense Ministry said in a statement.

The simplest way to bypass a missile defense system is to just launch more missiles than it can effectively intercept, says Jeffrey Lewis, director of the US-based East Asia Nonproliferation Program.

"Launching them simultaneously is more difficult for a missile defense system," he says. Monday's test involved three missiles, compared to just one test-fired on Kim Jong Un's birthday in April.

If those missiles are equipped with nuclear warheads "they don't have to get too many up in the air and past the missile defense system to have an effect," Lewis says.