Image copyright Getty Images Image caption Joe Biden is seen as Donald Trump's biggest rival in the Democratic race for the 2020 presidential election nomination

As the race for the 2020 presidential election gathers pace, the bitter war of words between US President Donald Trump and his main political rival Joe Biden is expected to escalate.

Seen as the frontrunner for the Democratic nomination, Mr Biden has been the main target of his rhetoric.

But some insults - on the face of it - go too far, even for President Trump.

On Sunday Mr Trump tweeted a rare, albeit backhanded, defence of Mr Biden after a North Korean attack on him.

North Korea had lambasted Mr Biden for having the "temerity to dare slander the dignity" of its leader, Mr Kim.

"Rabid dogs like Biden can hurt lots of people if they are allowed to run about," a statement, carried North Korea's official KCNA news agency, said on Thursday. "They must be beaten to death with a stick."

The president's response on Sunday included a retweet of a conservative commentator's post about North Korea's attack on Mr Biden.

Mr Trump's tweet was addressed to "Mr Chairman", an apparent reference to North Korea's leader Kim Jong-un.

The president did not hold back from his usual taunts against his US rival - calling him "sleepy and very slow" - but added that Mr Biden was "not a rabid dog", as North Korea had called him.

"He is actually somewhat better than that," Mr Trump added, while taking the opportunity to say he was the one who could best handle the situation.

It is not clear what drew the ire of North Korea, though Mr Biden has been critical of the Trump-Kim summits this year and last.

In response to the North Korean jibe, Mr Biden has said he saw such insults "as a badge of honour".

Media playback is unsupported on your device Media caption Watch the moment Donald Trump met King Jong-un and stepped foot inside North Korea

In contrast, Mr Trump's relationship with Mr Kim has been more amicable as he seeks to convince North Korea to give up its nuclear weapons through summitry rather than threats.

Mr Trump has lavished Mr Kim with compliments, describing him as "very sharp" and a "real leader".

Mr Biden, on the other hand, has frequently been on the receiving end of Mr Trump's jibes.

The impeachment inquiry, which centres on whether Mr Trump withheld military aid from Ukraine in exchange for an investigation into Mr Biden and his son, has intensified their long-running feud.

The clock is ticking. Kim Jong-un has given Donald Trump until the end of the year to come up with a denuclearisation deal before the North Korean leader says he will choose a "new path".

So this Trump tweet is timely.

It tells North Korea that the US is ready and eager to talk. "See you soon" even dangles the potential prize of another summit.

But Pyongyang is not making it easy. The two sides have only met once since Mr Trump and Mr Kim shook hands at the DMZ in June. The working-level negotiations in Stockholm in October were apparently cordial behind doors, but the North Koreans made a show of walking out and said the US had "disappointed them greatly".

Washington and Seoul have decided to postpone their annual air drills to give talks with North Korea a chance. These military exercises always anger the North and Pyongyang had warned of a "shocking punishment" if they went ahead. Calling off the drills will be seen by some as a major concession and may make the US look too eager to talk. But it does offer some breathing room for diplomacy to take place and together with Mr Trump's tweet, it sends the message to Kim Jong-un - it's your move now.