Nairobi (AFP) - At least 140 people have been killed in Ethiopia over the past two months in a crackdown on anti-government protests sparked by plans to expand the capital into farmland, Human Rights Watch said Friday.

"Security forces have killed at least 140 protesters and injured many more, according to activists, in what may be the biggest crisis to hit Ethiopia since the 2005 election violence," HRW's Felix Horne said.

The number reported by HRW is almost double the previous toll of 75 the group gave last month.

There was no immediate response from the Ethiopian government, which has previously put the death toll at five.

The protests began in November when students opposed government proposals to take over territory in several towns in the Oromia region, sparking fears that Addis Ababa was looking to grab land traditionally occupied by the Oromo people, the country's largest ethnic group.

"Over the past eight weeks, Ethiopia's largest region, Oromia, has been hit by a wave of mass protests over the expansion of the municipal boundary of the capital, Addis Ababa," Horne said.

"The generally peaceful protests were sparked by fears the expansion will displace ethnic Oromo farmers from their land, the latest in a long list of Oromo grievances against the government."

- Leaders and journalists arrested -

On December 23, police arrested Bekele Gerba, 54, deputy chairman of the Oromo Federalist Congress (OFC), Oromia's largest legally registered political party. Bekele was previously convicted in 2011 of being a member of the banned Oromo Liberation Front (OLF), spending four years in jail.

Bekele, a foreign language professor, "was reportedly hospitalized shortly after his arrest but his whereabouts are now unknown," HRW added. "Other senior OFC leaders have been arbitrarily arrested in recent weeks or are said to be under virtual house arrest."

The United States, a key ally of Ethiopia, last month expressed "grave concern" over the unrest. Washington has also criticised Ethiopia's arrest of journalists following the crackdown.

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The Committee to Protect Journalists (CPJ) say Fikadu Mirkana, from state-run broadcaster Oromia Radio and TV, and Getachew Shiferaw, editor of the Negere Ethiopia online newspaper, were arrested last month.

"By treating both opposition politicians and peaceful protesters with an iron fist, the government is closing off ways for Ethiopians to nonviolently express legitimate grievances," Horne said.

"This is a dangerous trajectory that could put Ethiopia's long-term stability at risk," he warned.

With at least 27 million people, Oromia is the most populous of the country's federal states and has its own language, Oromo, distinct from Ethiopia's official Amharic language.

HRW has said the protests -- and bloody crackdown -- echoed protests in April and May 2014 in Oromia, when police were accused of opening fire and killing "dozens" of protestors. The government said eight people died in the 2014 unrest.

Some 200 people were killed during post-election violence in 2005.