Arsenal midfielder Mikel Merino extended his remarkable scoring spree to six goals in the last four World Cup qualifying matches with two in the 4-0 win against Bulgaria

The 29-year-old proved the difference with a pair of headers on a night where his team-mates failed to take their chances.

Only Norway's Erling Haaland (12) and Netherlands' Memphis Depay (seven) have scored more in the current campaign.

The result maintained Spain's 100 percent record, having yet to concede, and kept them three points ahead of Turkey in Group E. It also equalled their all-time unbeaten run of 29 matches (excluding penalty shootouts).

Luis de la Fuente's side had scored 11 in their opening three matches and against pointless opponents who had let in 12 and scored just one it was evident which way the game was going to go.

The hosts had 18 shots in the first half - they finished the game with 30 - but only one was successful and that was Merino's 35th-minute header.

Pedri, who orchestrated things from midfield all night, chipped in a diagonal cross which could have picked out one of three team-mates but it dropped on the head of Robin Le Normand.

He directed it back into the path of his former Real Sociedad team-mate who nodded it forcefully past the goalkeeper.

Either side of that there were plenty of chances but none which properly tested Svetoslav Vutsov.

Álex Baena cut inside from the left but his attempt was routinely dealt with by the goalkeeper, who also twice denied striker Samu Aghehowa.

Pedri's chipped effort rebounded off the crossbar and just before Merino's breakthrough Mikel Oyarzabal saw his effort saved by Vutsov, who was not troubled by Aghehowa' third attempt of the night which flew wide.

There was no letting up after the break with Borja Iglesias, who replaced the wasteful Aghehowa at the break, twice extending Vutsov before Merino struck again.

Left-back Alejandro Grimaldo was given room to run down the flank and get to the byline where he crossed for Merino to head home powerfully from seven yards for his 10th goal for Spain.

That was enough for Spain but they continued to create and miss chances with Pablo Barrios firing just wide from the edge of the penalty area as Bulgaria, knowing the game was up, had their two best opportunities through Kiril Despodov and Radoslav Kirilov.

With Spain struggling to convert their dominance their opponents offered them a helping hand with Atanas Chernev turning Aleix Garcia's 79th-minute cross into his own net before Oyarzabal converted an added-time penalty.