TOKYO (Reuters) - Japan s navy is supplying fuel to U.S. ballistic missile defense (BMD) ships in the Sea of Japan, in a sign of deepening co-operation between the allies amid the growing threat from North Korea, a source said. By providing fuel to the U.S. Aegis destroyers as well as its own BMD ships, Japan hopes to ensure patrols can be maintained without unnecessary gaps, said the source, who has knowledge of the operation. The refueling began in April, the Nikkei financial daily reported earlier. North Korea threatened on Thursday to sink Japan with nuclear weapons for  dancing to the tune  of the United States for backing a U.N. Security Council resolution that imposed fresh sanctions on Pyongyang. The U.S. Navy s Seventh Fleet usually has around six Aegis ships assigned to BMD operations around Japan. Japan operates four ships of its own. They are armed with interceptors designed to shoot down warheads in space before they plunge to their targets. Around half of the ships would normally be at sea at any one time. Japanese Prime Minister Shinzo Abe s administration in 2015 won lawmaker approval to expand the role of the nation s Self Defence Forces (SDF) under the pacifist constitution to allow them to take on a bigger role in the alliance with Washington, including resupplying and defending U.S. ships. Japan has delivered fuel to U.S. ships in the past under ad hoc legislation, including vessels deployed to support military operations in Afghanistan a decade ago. But this is the first time it has been undertaken under the new security law. Japan s Chief Cabinet Secretary Yoshihide Suga at his regular morning press briefing on Thursday said that this kind of co-operation would be undertaken by the allies. He declined to comment on the Aegis ship fuel shipments saying  it could reveal details of SDF and U.S. Navy operations.  