TOP HEADLINES

· The U.S. would respond "decisively" if Russia uses nuclear weapons in Ukraine and there would be "catastrophic consequences" from an attack, warned National Security Advisor Jake Sullivan, talking to NBC. READ MORE BELOW

· Ukrainian President Volodymyr Zelenskyy said he doesn't think Vladimir Putin is bluffing when he says Moscow would use nuclear weapons to defend Russia."Look, maybe yesterday it was bluff. Now, it could be a reality," Zelenskyy told CBS News.

· Russian protests against mobilization have been backed by Zelenskyy in a video address. "Keep on fighting so that your children will not be sent to their deaths" he said. "Because if you come to take away the lives of our children - and I am saying this as a father - we will not let you get away alive."

· Voting in referendums for Ukraine regions over joining Russia has entered a fourth day on Monday. Russian-backed officials carry ballot boxes from door to door, accompanied by security officials, said Luhansk's regional governor Serhiy Gaidai and residents' names are taken down if they failed to vote correctly or refused to cast a ballot. READ MORE BELOW

· UK Prime Minister Liz Truss said: "We should not be listening to his (Putin's) sabre-rattling and his bogus threats. Instead, what we need to do is continue to put sanctions on Russia and continue to support the Ukrainians," in an interview with CNN.

· Vladimir Putin will meet his Belarusian counterpart Alexander Lukashenko on Monday in Moscow, Belarus state media reported. Russia and Belarus are close allies, with Russia having used bases in Belarus as a staging post for its troops, aircraft and equipment for its attack of Ukraine.

· Heavy fighting has seen more than 40 towns hit by Russian shelling in the past 24 hours, Ukraine officials have said. There have been five missile and 12 air strikes, as well as more than 83 attacks from multiple rocket-propelled grenades, the general staff of Ukraine's armed forces said.

· Two drones launched by Russian forces into Ukraine's Odesa region hit military objects, causing a fire and setting off ammunition, Ukraine's southern command said on Monday. "As a result of a large-scale fire and the detonation of ammunition, the evacuation of the civilian population was organized," it said on messaging app Telegram.

· Finland said on Monday it had recorded the year's busiest weekend in terms of Russians entering the country, after Moscow's military call-up announcement caused a rush for the border. "Last weekend was the busiest weekend of the year for traffic on the eastern border," said Mert Sasioglu of the Finnish border guard.

· Russian businessman Yevgeny Prigozhin said he founded the Wagner Group, a private military company, in 2014, the first public confirmation of a link he has previously denied. "I cleaned the old weapons myself, sorted out the bulletproof vests myself and found specialists who could help me with this. From that moment, on May 1, 2014, a group of patriots was born, which later came to be called the Wagner Battalion," Prigozhin said.

IN DETAIL

U.S. says it would act 'decisively' if nuclear weapons used in Ukraine

The U.S. warned Moscow of "catastrophic consequences" if it uses nuclear weapons in Ukraine after Russia promised protection to regions that might join it following referendums. U.S. National Security Adviser Jake Sullivan said the U.S. would respond to any Russian use of nuclear weapons against Ukraine.

"If Russia crosses this line, there will be catastrophic consequences for Russia," Sullivan told NBC television. "The United States will respond decisively."

Sullivan did not say how Washington would respond but said it had privately told Moscow "in greater detail exactly what that would mean".

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His comments followed Wednesday's thinly veiled nuclear threat by President Vladimir Putin, who said Russia would use any weapons to defend its territory.

Asked at the weekend if Moscow would consider using nuclear weapons to defend newly joined regions, Foreign Minister Sergei Lavrov said Russian territory, including that "further enshrined" in Russia's constitution in the future, was under the "full protection of the state".

Ukraine's President Volodymyr Zelenskyy said he did not believe Putin was bluffing when the Kremlin leader said Moscow would be ready to use nuclear weapons to defend Russia.

Referendum ballot boxes carried door to door

Citizens in four regions of Ukraine were voting for a fourth day on Monday in the Russian-organized referendums that Kyiv and the West have branded a sham. They say the outcomes are pre-determined and even countries with close ties to Moscow such as Serbia and Kazakhstan say they will not recognize the results.

But by incorporating the four regions - Luhansk and Donetsk in the east and Zaporizhzhia and Kherson in the south - Moscow could portray Ukraine's efforts to retake them as attacks on Russia itself, a warning to Kyiv and its Western allies.

The governor of Luhansk, Serhiy Gaidai, said Russian-backed officials were carrying ballot boxes from door to door, accompanied by security officials. Residents' names were taken down if they failed to vote correctly or refused to cast a ballot, he said.

"A woman walks down the street with what looks like a karaoke microphone telling everyone to take part in the referendum," the governor said in an interview posted online. "Representatives of the occupation forces are going from apartment to apartment with ballot boxes. This is a secret ballot, right?"

The four regions represent about 15 percent of Ukraine, or roughly the size of Portugal. Russian forces do not control all the territory in those regions, where fierce fighting still rages. They would add to Crimea, an area nearly the size of Belgium that joined Russia in 2014 after a similar referendum there.

Voting ends on Tuesday and Russia's parliament could then move swiftly to formalize the additional regions.