A Malaysian opposition party leader has called on Prime Minister Najib Razak to step down following the lawsuit filed by the US Justice Department to seize $1bn in assets linked to the country's scandal-plagued 1MDB state investment fund.

The Justice Department said on Wednesday that the assets were "associated with an international conspiracy to launder funds misappropriated" from 1MDB, and included lavish real estate in Beverly Hills and New York, artwork by Monet and Van Gogh, and a business jet.

"I believe the Malaysian people want Dato' Sri Najib to go on leave as prime minister so as not to create the perception of abuse of power or process to halt or hinder a full and transparent investigation on this very serious issue," Wan Azizah Wan Ismail, president of the People's Justice Party (PKR), said in a statement on Thursday.

Malaysia's government should allow an independent commission investigate corruption claims outlined by the Justice Department, said Wan Azizah, who is the wife of jailed Malaysian opposition leader Anwar Ibrahim.

The lawsuit alleges a complex money laundering scheme that the Justice Department said was intended to enrich top-level officials of 1MDB.

In a press conference in Washington, DC, on Wednesday, US Attorney General Loretta Lynch said the 1MDB funds were used as a "personal bank account".

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The fund is owned by the Malaysian government, but none of the lawsuits named Prime Minister Najib Razak, who has consistently denied any wrongdoing.

However, the case named Riza Aziz, the prime minister's step-son, as a "relevant individual" in the case.

The lawsuits also named Najib's friend, Malaysian financier Low Taek Jho, or Jho Low, and Abu Dhabi government officials Khadem al-Qubaisi and Mohamed Ahmed Badawy Al-Husseiny.

Al Jazeera's correspondent Sohail Rahman, speaking from Kuala Lumpur, said the prime minister's office issued a statement overnight in response to the allegations.

As the prime minister holds the financial portfolio under which 1MDB operates, and many of his critics say he should have known what was going on "and many actually accuse him of being involved in this whole scenario," Rahman said.

Ordinary Malaysians will likely be shocked by this US investigation, as the Malaysian government's own probe of 1MDB has already ended, he said.

"The case in theory has been closed since October when the attorney general here in Malaysia said that there was no wrongdoing, and ordered the Malaysian anti-corruption commission to close the case."

"However, this re-opens it from, certainly, across the Pacific, where the US now will go forward to try and investigate how these assets were bought," Rahman said.

In the statement, the prime minister's office said it would "fully cooperate with any lawful investigation of Malaysian companies or citizens, in accordance with international protocols, as the prime minister has always maintained, if any wrongdoing is proven, the law will be enforced without exception," he added.

'Money-laundering scheme'

The 1MDB fund was created in 2009 by the Malaysian government with the goal of promoting economic development projects in the Asian nation.

Instead, officials at the fund diverted more than $3.5bn over the next four years through a web of shell companies and bank accounts in Singapore, Switzerland, Luxembourg and the US, according to the justice department complaint.

Federal officials said more than $1bn was laundered into the US for the personal benefit of 1MDB officials and their associates.

The funds were used to pay for luxury real estate in the US and Europe; gambling expenses in Las Vegas casinos; a London interior designer; more than $200m artwork by artists, including Van Gogh and Monet; and for the production of films, including the 2013 Oscar-nominated movie "The Wolf of Wall Street".

The complaint said that among those who profited from the scheme was the prime minister's step-son Aziz, who co-founded Red Granite Pictures, a movie production studio whose films include "The Wolf of Wall Street".

According to the complaint, 11 wire transfers totaling $64m were used to fund the studio's operations, including the production of the movie starring Leonardo DiCaprio.

Leslie Caldwell, US assistant attorney general, said at the news conference on Wednesday that neither 1MDB nor the Malaysia people saw "a penny of profit from that film," or the other assets that were purchased with fund siphoned from 1MDB.

Red Granite said on Wednesday that none of the funding it received four years ago was illegitimate and nothing the company or Riza did was wrong.

Authorities in neighbouring Singapore also announced on Thursday that they seized assets worth $240m in their own investigation of 1MDB-related fund for possible money laundering.

The Monetary Authority of Singapore, as well as the city state's Attorney-General's Chambers and the Commercial Affairs Department said their investigation of the funds found "deficiencies" at several major banks, including "undue delay in detecting and reporting suspicious transactions."

Al Jazeera’s correspondent in Washington DC Kimberly Halkett said the investigation is likely to strain US relations with Malaysia, which President Obama had personally tried to cultivate, having played a round of golf with Najib during a visit to the country in 2014.