Lewis McNall's penalty gave the Magpies an early lead in front of a crowd of over 1,000 at St. James' Park, and Dave Watson's side were indebted to goalkeeper Nathan Harker's penalty save from Keiran Evans as they held their advantage until half time.

Liam Cullen's close-range finish drew the Swans level just after the break, but Allan's header restored the hosts' lead before his last-minute breakaway goal made sure of their place in the Fifth Round.

Barely 20 seconds into the tie, United had a  sight of goal when Matty Longstaff's well-weighted through ball found Callum Smith, but the winger couldn't get a shot away and Swansea retreated to clear. 

But they didn't have to wait much longer for an opener. Owen Gallacher broke into the box at speed and, when he went down under the challenge of a Swans defender, referee Billy Khatib pointed to the spot. Forward McNall stepped up to do the honours, sending goalkeeper Ross Treacy the wrong way to give the hosts a third-minute lead.

Ten minutes later, the visitors had their first real sight of goal when Liam Edwards picked up a loose ball and shot, but a superbly-timed sliding challenge from Lewis Gibson denied him.

At the other end, Gallacher saw his shot blocked after he gathered a Smith cross at the back post and Longstaff whipped a low free kick just past the upright, but the Swans retained a threat, and Oliver Cooper dragged a shot past the post from just inside the area.

And in the dying seconds of the half, Swansea were handed a golden chance to level the scores when referee Khatib adjudged Mace Goodridge to have brought down Edwards in the area. Evans stepped up to take it, but Harker went the right way and held on to the tame spot-kick to preserve United's lead going into the break.

Seven minutes into the second period, though, Swansea did draw level. When Newcastle couldn't clear a corner from the left, Cullen - who bagged two in their victory over Wolves in the Third Round - pounced to apply the finishing touch from close range after a scramble in the goalmouth.

With Swansea in the ascendency, Watson's side were forced to weather the storm in the minutes after the equaliser, with Harker saving at the feet of substitute Causso Darame and then clutching Ben Cabango's header.

But Newcastle responded superbly, and regained the lead when Allan, who replaced Mo Sangare at half time, nodded home Longstaff's inswinging free kick from the left to restore the lead with just over 23 minutes left on the clock.

As legs began to tire, Adam Wilson and Lewis Cass were introduced from the bench as the Magpies sought to preserve their lead and with goalkeeper Treacy stranded upfield as the visitors pushed for another equaliser, Allan latched on to Harker's punch to sprint clear from his own half and finish into an empty net to make sure of the win.

Newcastle will now face a trip to either local rivals Sunderland or Shrewsbury Town in the Fifth Round.

Newcastle United Under-18s: Nathan Harker, Craig Spooner, Kieren Aplin (Lewis Cass 77), Matty Longstaff, Owen Bailey, Lewis Gibson, Owen Gallacher (Adam Wilson 75), Mace Goodridge, Lewis McNall, Mo Sangare (Thomas Allan 46), Callum Smith.

Subs not used: Otto Huuhtanen, Daniel Lowther.

Swansea City Under-18s: Ross Treacy, Mael Davies, Ben Ericksen (Joe Lewis 62), Tom Price, Ben Cabango, Brandon Cooper, Oliver Cooper, Cameron Berry (Mason Thomas-Jones 75), Liam Cullen, Keiran Evans (Causso Darame 56), Liam Edwards.

Subs not used: Alex Lang, Marco Dulca.

Referee: Billy Khatib

Yellow cards: Newcastle United Under-18s - Owen Gallacher (37); Swansea City Under-18s - Ben Cabango (75)