Donald Trump on Saturday said he feels "sharper now than I did 20 years ago," reacting to recent attacks on his age and verbal gaffes by Republican presidential rival Nikki Haley.

Trump also said presidential candidates should have to take a cognitive test, apparently a response to a challenge from Haley, who has advocated the same policy, citing the age of Trump, 77, and Democratic President Joe Biden, 81.

Trump was speaking at a rally in Nevada, ahead of the next vote in the Republican presidential nominating race, a caucus in the state on Feb 8. In recent days Haley has accused former Republican President Trump of being confused and has questioned his ability to be president at his age.

Trump has recently made some verbal slip-ups. During a speech on Jan 19 he confused Haley with former Democratic House Speaker Nancy Pelosi. On occasion he has appeared to slur his words and he also suggested former Democratic President Barack Obama was still in office.

Trump is almost assured of all of Nevada's 26 delegates because Haley is not competing in the caucus. He attacked both Haley and Biden, trying to knock Haley out of the Republican nominating fight while scoring early points in a likely general election rematch with Biden in November.

Trump and his allies have begun a campaign to force Haley out of the race before the next major vote in the primary race, in her home state of South Carolina on Feb. 24. Trump has threatened to banish from his political orbit any donors who continue to fund Haley.

Haley has pledged to keep campaigning in South Carolina and beyond.

Shortly after Trump's speech, Haley appeared at a rally in South Carolina. She again said Trump had recently appeared "confused", and said if he wants to take a cognitive test, "he should have no problem getting on a debate stage with me, because that's the ultimate mental competency test for anyone running for president."

Trump has refused to participate in any of the Republican nominating debates and is refusing to debate Haley.

In his Nevada speech, Trump repeated a demeaning nickname for Haley, calling her "birdbrain". He also accused Haley - a conservative Republican - of being "almost a radical left Democrat".

Trump declared: "It's time to finish this," referring to his nomination fight. He leads Haley in opinion polls in South Carolina and she has no clear path to the nomination.

After Trump's win in the New Hampshire primary on Jan 23, the Biden campaign issued a statement, saying: "It is now clear that Donald Trump will be the Republican nominee."

Turning his attention to Biden, Trump focused much of his speech on the southern border.

Record numbers of migrants have been caught illegally crossing the US-Mexico border since Biden took office in 2021, and opinion polls show immigration and the border as a top issue in this year's general election.

Trump called the illegal crossing a "catastrophe", an "invasion" and the southern border as an "open wound".

Meanwhile, Biden and his campaign aides have intensified attacks on Trump in recent days, calling him a threat to US democracy and tying him to the US Supreme Court's 2022 decision to end federal abortion rights, an issue that has been blamed with hurting Republicans during the 2022 midterm elections. 

At least 34 people, including two babies were killed when a contraband fuel depot exploded into flames in southern Benin near the border with Nigeria on Saturday.

According to Beninese Interior Minister Alassane Seidou, “a serious fire occurred in the town of Seme Podji. Another 20 people were seriously wounded in the incident.”

"I can't really give you the cause of the fire, but there is a large gasoline warehouse here and cars, tricycles and motorcycles come from morning to evening,” he added.

Nigeria is a main oil and gas producer where fuel smuggling is very common along its borders, particularly when the government maintains a subsidy to keep fuel prices low.

The majority of the victims were burnt and they could not be not be identified.

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