WASHINGTON (Reuters) - The Trump administration on Wednesday imposed sanctions against two senior South Sudanese officials and the country’s former army chief in a warning to the government of President Salva Kiir over increasing attacks on civilians in the country’s four-year civil war.

The U.S. Treasury Department in a statement on its website said it had blacklisted Malek Reuben Riak Rengu, deputy chief of defense for logistics in South Sudan’s army; Paul Malong, former army chief sacked by Kiir in May; and Minister of Information Michael Makuei Lueth for their roles in destabilizing South Sudan.

The measures freeze any assets in the United States or tied to the U.S. financial system belonging to the three men.

Mawien Makol, spokesman at South Sudan’s foreign affairs ministry, called Washington’s announcement unfortunate.

“Such sanctions can undermine the efforts rather than help the efforts,” Makol said, referring to a 2015 peace deal.

Nathaniel Oyet, a senior official in the opposition SPLA-IO group, welcomed the move although added: “It has come a bit late. We wanted it yesterday.”

“This now gives us the confidence that the Donald Trump administration will fix the crisis in South Sudan,” said Oyet.

The U.S. crackdown comes days after Trump’s new aid administrator, Mark Green, visited South Sudan to deliver a blunt message to Kiir that Washington was reviewing its policy toward his government. He called on Kiir to end the violence and implement a “real” ceasefire.

The meeting signaled that the Trump administration was reconsidering its backing for Kiir, who came to power with the support of Washington when oil-rich South Sudan won independence from neighboring Sudan in 2011 following decades of conflict.

But the world’s youngest country dissolved into civil war in 2013 after Kiir, an ethnic Dinka, fired his deputy Riek Machar, a Nuer. Nearly one-third of the country’s population - or 4 million people - have fled their homes, creating the continent’s largest refugee crisis since the 1994 Rwandan genocide.

In its statement, the U.S. Treasury said Malek Reuben was central to weapons procurement during the first few years of the conflict and helped plan an offensive in Unity State in April 2015, which targeted civilians and led to “numerous rights abuses.”

It also accused him of issuing military contracts at inflated prices “in order to receive extensive kickbacks.”

The U.S. Treasury blacklisted All Energy Investments, A+ Engineering, Electronics & Media Printing and Mak International Services which it said was owned or controlled by Malek Reuben.

Malong was sacked by Kiir in May as army chief and put under house arrest in the capital Juba, the country’s defense minister told Reuters last week.

The U.S. Treasury said he was being sanctioned for obstructing peace talks, international peacekeeping efforts and humanitarian missions in South Sudan.

The Treasury statement said Malong was reportedly responsible for efforts to kill Machar in 2016 and “did not discourage” the killing of civilians around the town of Wau last year.

It said Malong was found with “currency worth millions of U.S. dollars in his possession” belonging to the military’s treasury as he tried to flee Juba in early May.

Reached by phone in Nairobi, Malong’s wife, Ayak Lucy, told Reuters her husband did not have financial assets in the United States. She was unaware of the sanctions announcement.

Meanwhile, the U.S. Treasury accused Makuei of attacks against the U.N. mission in South Sudan and obstructing of peacekeeping and humanitarian missions in the country.

“What do they call it? Economic sanctions? What property do I have in America and all over the world?” he told Reuters in response to the sanctions.