Ongoing air drills with South Korea will "sharpen" their joint combat capabilities, the US military said Thursday, as the nuclear-armed North ramps up threats and a balloon blitz against Seoul.

Seoul and Washington's air forces started around three weeks of joint drills Tuesday in Suwon, south of Seoul, involving US F/A-18 and F-35B combat aircraft.

The drills "will further sharpen our combat capabilities", US Marine Lieutenant-Colonel Jarrod Allen said in a statement.

Joint U.S.-South Korea drills typically infuriate Pyongyang, which views them as rehearsals for invasion, and the North is particularly sensitive to fighter jet exercises as experts say its airforce is the weakest link in its military.

Relations between the two Koreas are at one of their lowest points in years, with the North sending thousands of trash-carrying balloons southwards and Seoul's military blasting K-pop and anti-regime messages from border loudspeakers.

The North said Wednesday it was "fully ready for all-out confrontation with the US", responding to comments by former President Donald Trump, the Republican presidential nominee for the November election, touting his ties to Kim Jong Un.

Trump said "I think he misses me" and it's "nice to get along with somebody that has a lot of nuclear weapons."

While in office Trump met with Kim three times, beginning with a landmark summit in Singapore in June 2018, but the pair failed to make much progress on efforts to denuclearize the North.

A few months after Singapore, Trump famously told a rally of his supporters that the two men had fallen "in love."

But their second summit in Hanoi collapsed in 2019, over sanctions relief and what Pyongyang would be willing to give up in return.

In a commentary released Wednesday, North Korea said while it was true Trump tried to reflect the "special personal relations" between the heads of states, the former US president "did not bring about any substantial positive change."

"Even if any administration takes office in the US, the political climate, which is confused by the infighting of the two parties, does not change and, accordingly, we do not care about this," it added.