Hungarian Prime Minister Viktor Orban said Serbian President Aleksandar Vucic had informed him by phone about the discovery, which they said had taken place outside the town of Kanjiza, near Hungary's border with Serbia.
"Our units found an explosive of devastating power," Vucic, a close ally of Orban, said in a post on Instagram.
"I told PM Orban that we would keep him updated on the investigation."
They did not give further details of the explosives or publish images of the discovery on a section of pipeline linked to the Turkstream system, which carries Russian gas to Turkey and then to Central Europe.
Officials in Budapest and Belgrade did not respond to requests for comment about the incident and questions surrounding it, which comes a week before pivotal elections on April 12 in Hungary.
Nationalist Orban is fighting to hold onto his more than 16-year grip on power in the election, with his party trailing the opposition Tisza party in polls.
A former Hungarian intelligence official told Reuters there had been discussions in Hungarian security circles over the past days about a precise plan for a "false-flag" operation impacting the pipeline in Serbia as part of an effort to influence the Hungarian vote.
Tisza leader Peter Magyar also raised doubts about the leaders' statements, saying they appeared aimed at boosting Orban's electoral prospects.
"Several people have publicly indicated that something will 'accidentally' happen at the gas pipeline in Serbia at Easter, a week before the Hungarian elections. And so it happened," Magyar said in a statement.
Orban said in a Facebook post that he had called a defence council meeting on Sunday to discuss the incident on the pipeline, which transports Russian gas through the Balkans to Central and Eastern Europe.
"According to information that we have....there was an act of sabotage prepared," Orban said after the meeting, adding that both countries have strengthened the protection of the pipeline.
Without directly blaming Ukraine for the incident in Serbia, Orban said: "Ukraine has been for years trying to cut off Europe from Russian energy."
"The Russian section of TurkStream is also under continuous military attack. Ukraine's efforts pose a life-threatening danger to Hungary," he added.
Orban in February scaled up security around energy infrastructure in the country by dispatching troops after what he said were plans by Ukraine to disrupt the Hungarian energy system - charges Kyiv denied.
Budapest has also been in a dispute with Ukraine over a halt in oil supplies via the Druzhba pipeline.
Orban's Fidesz party has sought to associate opposition leader Peter Magyar with Brussels and Ukraine, suggesting that voting for his Tisza party means voting for tanks and war.
Ukraine's foreign ministry strongly rejected what it said were attempts to link Kyiv to the explosives.
"Ukraine has nothing to do with this," foreign ministry spokesman Heorhii Tykhyi said on X. "
Most probably, a Russian false-flag operation as part of Moscow's heavy interference in Hungarian elections."
Hungary is an outlier in the European Union for maintaining ties with Moscow, which voiced support for Hungary over Sunday's incident and suggested that Ukraine was responsible.
"(Ukraine) wants to deprive Hungary of its sovereignty," Russian Foreign Ministry spokesperson Maria Zakharova told the TASS news agency.
Hungarian Foreign Minister Peter Szijjarto said in a post on Facebook that "in the past few days and weeks, the Ukrainians organised an oil blockade against us, and then tried to put us under a total energy blockade ... And now we have today's incident."
