By Shihar Aneez and Ranga Sirilal

COLOMBO (Reuters) - The speaker of Sri Lanka's parliament accepted on Tuesday a ruling party motion to impeach the chief justice for improper conduct, setting the stage for a potentially destabilizing clash between the government and judiciary.

Relations between President Mahinda Rajapaksa and Chief Justice Shirani Bandaranayake have been deteriorating over recent months with the judiciary complaining of interference and the government complaining she had over-stepped her authority.

Bandaranayake, Sri Lanka's first woman chief of the Supreme Court, faces fourteen charges ranging from undeclared assets to violating constitutional provisions, according to the impeachment motion lodged in parliament.

"Yes, he has accepted it," parliament Secretary General Dhammika Dassanayake said, confirming that the motion had been officially received by the speaker, Chamal Rajapaksa, the president's brother.

The president appointed Bandaranayake in May last year but their relationship has soured swiftly this year. Rajapaksa's party moved the impeachment motion in parliament last Thursday.

The next day, the United States raised its concern.

"We urge the government of Sri Lanka to avoid any action that would impede the efficacy and independence of Sri Lanka's judiciary," U.S. State Department spokeswoman Victoria Nuland said in a statement.

The International Commission of Jurists, grouping 60 prominent judges and lawyers from around the world, said last week the judiciary in Sri Lanka was under increasing attack and its independence was threatened.

The speaker will appoint a select committee made up of at least seven members to look into the charges against Bandaranayake.

She could be removed with a simple majority of 113 votes in the 225-member parliament. Rajapaksa and his allies control more than two-thirds of seats. The motion has been signed by 117 ruling party legislators.

"NOT BACK DOWN"

Bandaranayake's supporters say she has been trying to preserve the independence of the judiciary in a highly politicized environment.

But she came under criticism from the government after she ruled against a bill in parliament proposing a budget of 80 billion rupees ($614.20 million) for development.

She said the bill had to be approved by the country's nine provincial councils, including the former war-torn northern province, before being approved by parliament.

The block on the bill angered the government and its supporters, some of whom accused the judiciary under Bandaranayake of over stepping its limits and violating the constitution.

The chief justice has not spoken to the media but her son, Shaveen Bandaranayake, issued a statement last week saying his mother stood for the independence of the judiciary and democracy.

"My mother will not back down," he said.

The impeachment bid comes as international human rights call for an independent inquiry into the deaths of civilians and aid workers during Sri Lanka's three-decade civil that ended in 2009.

A U.N.-sponsored panel, whose findings have been rejected by the Sri Lankan authorities, has said that the army committed large-scale abuses and was responsible for many civilian deaths in the final stages of the war against ethnic Tamil rebels.

(Editing by Robert Birsel)