USA Correspondent: US President Donald Trump is delivering his annual State of the Union address before Congress.

The Republican president called for political unity, hours after using an offensive term to describe the Democratic leader of the US Senate.

He will also use the speech to address border security, and talk about North Korea and the trade dispute between the US and China.

In a rebuttal, Democrats will accuse Mr Trump of abandoning US values.

Mr Trump's primetime address follows the longest US government in history over border security.

He provoked the crippling closure of federal agencies by demanding funding for a US-Mexico border wall only to backtrack when Democrats flatly refused.

What did he say about political unity?
After two years of toxic partisanship, Mr Trump on Tuesday night repeated calls for political unity that he has made in his previous two State of the Union speeches.

"Together, we can break decades of political stalemate," he said.

"We can bridge old divisions, heal old wounds, build new coalitions, forge new solutions, and unlock the extraordinary promise of America's future. The decision is ours to make."

But in an apparent reference to Democratic accusations that his campaign colluded with Russia during the 2016 election campaign, Mr Trump criticised "ridiculous partisan investigations".

Hours before the speech, Chuck Schumer, the Democratic leader of the Senate, poured pre-emptive scorn on Mr Trump's remarks.

The New York senator said on the chamber floor: "It seems every year the president wakes up and discovers the desire for unity on the morning of the State of the Union, then the president spends the other 364 days of the year dividing us, and sowing a state of disunion."

The president fired back on Twitter: "I see Schumer is already criticizing my State of the Union speech, even though he hasn't seen it yet."

In a private lunch with news anchors at the White House on Tuesday, Mr Trump called Mr Schumer "nasty", using an offensive term, reports the New York Times.

The president was also quoted as having called former US Vice-President Joe Biden, a potential White House challenger in 2020, "dumb".

As Mr Trump delivered his primetime speech on Tuesday, his chief congressional antagonist was sitting at the rostrum over his shoulder.

The Democratic leader of the House of Representatives, Speaker Nancy Pelosi, led congressional resistance to the president's demands for wall funding and regularly mocks him.

How will Democrats respond?
Stacey Abrams, who lost her race last year to be governor of Georgia, will deliver the Democrats' response to Trump, making her the first African-American woman to deliver the party's rebuttal.

She was expected to say: "The shutdown was a stunt engineered by the president of the United States, one that defied every tenet of fairness and abandoned not just our people - but our values."

A number of Democratic female lawmakers - who were wearing white, the colour favoured by early 20th Century suffragettes - sat stony faced as Republicans gave repeated ovations for the applause lines.