The Sweden striker scored twice in the opening half, first levelling from the spot after Jan Bednarek's headed opener, before firing past Alex McCarthy to fire Eddie Howe's side ahead on the half-hour mark.

That made Isak the first player in Newcastle's history to score in five consecutive Premier League away games, with his double proving decisive against Ivan Juric's relegation-threatened hosts.

Sandro Tonali added a third after the interval after fine link-up play with Isak and Anthony Gordon, while Mateus Fernandes saw a late strike ruled out for offside after a video assistant referee (VAR) review.

It may have been a different outcome, however, if VAR had not recommended an on-field review after Joe Aribo sliced down Isak in the first half, with a penalty awarded after a lengthy check by Samuel Barrott.

The win lifted Newcastle back into fourth, one point and place clear of Chelsea even before its loss to Manchester City later in the day, while Southampton remains 10 points adrift of safety at the bottom.

Newcastle boss Eddie Howe that gritty resilience was vital in moving his team up the Premier League table.

"It was a tough game, as we knew it would be. Southampton came out quickly and scored, so that changed the dynamic of the game. We needed to really dig deep, it was a big test of our character, especially after what happened last week [against Bournemouth]," Howe said.

"We needed a big response, and we got that with the penalty, so we never looked back.

"I'm so pleased for Isak [to score again] He was the conduit for us in this game, he was the guy linking play and then on the end of bits and pieces. He took the second goal really well and it was a great move from the team. I'm delighted with him, delighted for the group. It's a big win for us."

Next up for Howe is a home clash with Fulham next weekend, followed by a crunch Carabao Cup semi-final second leg at home to Arsenal with the Magpies holding a 2-0 advantage as they aim for a second final appearance in the competition in three years.

Another loss means Southampton has now lost all six of Juric's Premier League games in charge, as it remains rooted to the foot of the table with just six points overall, and a 10-point gap to safety.

The Saints face a monumental challenge to try and remain in the top-flight next season, but Juric admitted at full-time that he could not ask anything more of his players, as they simply came up short against a superb Newcastle reaction.

"Today my guys gave everything they have. They are fighting well, playing well, creating chances. Of course, Newcastle were the better team. I don't have many negative things to say to my guys," he said.

"The situation is really bad, but I want to fight and create something here. I saw some good moments from the team today. But, we have to understand really well which players are ready for the Premier League."

Southampton heads to fellow relegation battler Ipswich Town next weekend, as it look for a first away win of the league campaign, with the unwanted record of the lowest Premier League points total still on the cards for Juric, a record currently held by Derby County's 11 points from 2007-2008.