The party of Malaysia's scandal-tainted Prime Minister Najib Razak has won two parliamentary by-elections in a landslide, further strengthening his rule despite corruption allegations.

Najib's United Malays National Organization retained the Sungai Besar seat in central Selangor state and Kuala Kangsar in northern Perak state in the elections Saturday with much larger majorities. The elections were triggered after the deaths of the incumbents in a helicopter crash.

The victory was expected due to a fractured opposition, as well as the ruling coalition's well-oiled machinery and money. Voters in the two rural constituencies are mostly ethnic Malays, the bedrock of support for Najib's Malay party.

Najib said the victory showed that the people rejected "politically-motivated slander" against his government. Last month, he also secured a major win for the coalition in a state election.

"This will significantly strengthen his position in UMNO. Najib can claim that he can lead the party and the coalition to electoral victories despite all the scandals," said James Chin, director of the Asia Institute at Australia's University of Tasmania.

Najib has been grappling with accusations of corruption and mismanagement over allegations that hundreds of millions of dollars had been channeled into his accounts from an indebted state fund.

Under pressure to resign, Najib has denied the money came from state investment fund 1MDB. The government cleared him in January, saying the money was a donation from the Saudi royal family and that Najib had returned most of it. The explanation was met with widespread skepticism.

The fund, which Najib started in 2009, is still mired in debt and is being investigated in several countries for embezzlement. A parliamentary inquiry found massive unexplained payments and called for a police investigation of the fund's former head.