By David Brunnstrom, Emma Farge and Olivia Le Poidevin

April 21 (Reuters) - The four candidates vying to become the next United Nations secretary-general face live hearings on Tuesday and Wednesday on their bids to lead the troubled global organization from next year.

Chile's Michelle Bachelet, Argentina's Rafael Grossi, Costa Rica's Rebeca Grynspan and Senegal's Macky Sall are competing for a five-year term at the helm of the 193-member body, which can be extended for another five.

GENEVA, April 21 (Reuters) - The 10th United Nations secretary-general will be elected this year for a five-year term starting on January 1, 2027.

Here are the candidates so far running to take over from outgoing U.N. chief Antonio Guterres.

RAFAEL GROSSI

Rafael Grossi, a 65-year-old career diplomat from Argentina, has been an omnipresent, hyperactive director general of the U.N. nuclear watchdog for the past six years.

Oil prices climbed and stocks slid Tuesday as uncertainty reigned over US-Iranian peace talks resuming, and the expiration of the two-week ceasefire in the Middle East loomed.

Shortly after the US stock market closed, US President Donald Trump announced that he was extending the ceasefire, while keeping in place a military blockade on Iran's ports.

Investors had been hoping that despite hostile rhetoric there was scope for a deal to end the Middle East war and reopen the vital Strait of Hormuz to oil and gas shipments.

By Mike Stone

WASHINGTON, April 20 (Reuters) - The U.S. Air Force secretary extended the life of the A-10 "Warthog" attack plane until 2030, sparing the aging but beloved close air support aircraft that has played an important role in Iran from an earlier retirement deadline of 2026.

"We will EXTEND the A-10 'Warthog' platform to 2030," Air Force Secretary Troy Meink posted on social media, adding the move "preserves combat power as the Defense Industrial Base works to increase combat aircraft production."

By Samia Nakhoul

DUBAI, April 20 (Reuters)- - A warning by former Russian president Dmitry Medvedev has crystallised fears among Gulf states that reopening the Strait of Hormuz may be the most Iran-U.S. talks can achieve, falling short of the broader de-escalation they regard as vital.

Officials and analysts expect the next round of negotiations, due in Islamabad, will focus increasingly not on Iran's missiles or regional proxies but on uranium enrichment limits and how to handle Iran’s leverage over the Strait, the world’s most critical oil shipping route.

April 20 (Reuters) - Pakistan is confident it can get Iran to attend talks with the United States, a senior Pakistani government official told Reuters on Monday.

"We have received a positive signal from Iran. Things are fluid but we are trying that they should be here when we start the talks tomorrow or a day after," the source said, speaking on condition of anonymity.

The source added Pakistan is actively engaged with Tehran and Washington as U.S. President Donald Trump's ceasefire deadline looms.

WASHINGTON, April 20 (Reuters) - President Donald Trump said on Monday he believed a nuclear deal the U.S. is currently negotiating with Iran will be better than a 2015 international agreement to curb Tehran's nuclear program.

"The DEAL that we are making with Iran will be FAR BETTER than the JCPOA, commonly referred to as 'The Iran Nuclear Deal',” Trump wrote in a social media post after concerns expressed by Democrats and some nuclear experts that he is rushing negotiations on a highly complex topic.

By Lili Bayer

BRUSSELS, April 20 (Reuters) - The Board of Peace’s lead envoy for Gaza told Reuters on Monday that he was “fairly optimistic” a plan for disarmament of Hamas and other militant groups in Gaza can be agreed but cautioned that it will still take time.

“We've had some very serious discussions with Hamas over the last few weeks, they're not easy,” Nickolay Mladenov said in an interview during a visit to Brussels.

ANKARA, April 20 (Reuters) - Turkey will tighten gun-ownership rules and increase penalties for owners whose children find the firearms, President Tayyip Erdogan said on Monday after last week's rare, deadly school shootings.

The second of the two separate attacks by students last week killed eight pupils and a teacher in a southeastern province of Kahramanmaras, shocking a nation in which such school-based violence is very rare.

Erdogan said the government will also take additional steps to monitor the internet as part of its response to the incidents.

Iran is banking on the Strait of Hormuz as its main card in any peace negotiations with the United States, but using the waterway as leverage is not without risk for the Islamic republic.

Iran had for decades talked up the threat of blocking the strait, which is a critical chokepoint for global energy supplies.

With the US and Israel launching a surprise war on Iran on February 28 and killing supreme leader Ali Khamenei, Tehran finally made good on those threats and brought the vital waterway to a standstill.