The government will pledge in key security documents to be revised this month to acquire a "counterstrike capability," but exclusively for defense purposes under the war-renouncing Constitution, a government source has said.

The revision to the National Security Strategy and two other documents, including the controversial idea of allowing the country to fire upon and disable enemy missiles before they are launched from foreign territory, comes amid North Korea's progress with its nuclear program and as China increases its military buildup.

While the mention of acquiring an enemy base strike capability heralds a major shift in its security policy, Japan will pledge in the documents to continue "to commit to its exclusively self-defense-oriented security policy" and "to not become a military power," the source said.