WASHINGTON -- The United States and its allies stand ready to defend themselves even on Christmas day, a Pentagon spokesman said Wednesday, as officials remained on alert for a threatened "Christmas gift" from North Korea.

The "gift" was widely expected to be an intercontinental ballistic missile launch after Pyongyang twice conducted apparent rocket engine tests at its western satellite launch site earlier this month.

The US flew four surveillance planes over the Korean Peninsula between Tuesday and early Wednesday in an indication of heightened preparations to respond to a North Korean provocation.

"The US, along with our partners and allies throughout the world, stand ready to defend ourselves, even on Christmas Day," Pentagon spokesman Lt. Col. Dave Eastburn told Yonhap News Agency without elaborating.

A long-range missile test would be a setback for Donald Trump as the US president has claimed the North's suspension of such launches as a major diplomatic victory.

North Korean leader Kim Jong-un declared a moratorium on nuclear and ICBM tests in April 2018 ahead of his first summit with Trump in Singapore in June that year.

The two leaders have met three times in a bid to strike a deal on denuclearizing North Korea in exchange for US sanctions relief and security guarantees, but negotiations have stalled amid wide differences over how to match their steps.

In a sign of Pyongyang's growing impatience, the regime has test-fired multiple short-range missiles and other rockets since May and threatened to deliver a "Christmas gift" to the US short of concessions from Washington.

Speaking to reporters at his Mar-a-Lago resort in Florida on Tuesday, Trump brushed off the warning and quipped that the gift could be a "beautiful vase."

"We'll find out what the surprise is and we'll deal with it very successfully," he said. "Maybe it's a nice present. Maybe it's a present where he sends me a beautiful vase as opposed to a missile test. I may get a vase. I may get a nice present from him. You don't know. You never know."

South Korea and the US have strengthened vigilance against the possibility of North Korea firing an ICBM or a submarine-launched ballistic missile, or undertaking other types of provocations, military sources in Seoul said.

Green Pine land-based radars were in operation on the ground, while an Aegis destroyer equipped with the SPY-1D radar system was deployed off shore and an E-737 Peace Eye airborne early warning and control aircraft was carrying out a mission in the air, according to the sources.

"We're keeping a close watch over military moves in North Korea," one source said. (Yonhap)