Titanium dioxide (TiO2) producer Tronox is suing the US Federal Trade Commission (FTC), alleging that the antitrust regulator is trying to illegally block its proposed acquisition of Saudi Arabia’s Cristal.

In the latest twist of the ongoing wrangle between the two, Tronox has filed a lawsuit in the US District Court for the Northern District of Mississippi seeking a declaration and injunction that it said would prevent the FTC using its administrative process as a delaying tactic until the agreement with Cristal expires in May.

“Rather than follow its long-established practice to file suit in federal court to block an acquisition, the FTC has engaged in a strategy of delay by initiating an administrative process that offers no possibility of a timely resolution,” said Richard Muglia, senior vice president and general counsel at Tronox.

Muglia said that the FTC’s refusal to allow a federal court to address the pro-competitive merits of the transaction denied Tronox any meaningful opportunity to show how customers could benefit from the combined group’s ability to capture significant synergies and increase production, enabling it to better compete with global market leaders and lower-cost Chinese producers who continue to strengthen their presence.

The pigment manufacturer originally announced the $2.4 billion takeover in February 2017 and filed its first notification form with the FTC on Mar. 14. The waiting period has been extended several times by agreement of both parties.

Last December, the FTC filed a complaint in a bid to stop the merger, alleging it would reduce competition for chloride-based TiO2 as both companies are the top two suppliers of this particular grade.

Tronox disagreed, saying that the FTC’s view of the global TiO2 market was incorrect and ignored sulfate-based pigment, and its analysis of the transaction was flawed.

If the acquisition is not completed by May 21, 2018, either party can terminate the agreement unless the date is amended by mutual consent.

Earlier this month, Australia, China, New Zealand, Turkey, South Korea and Colombia gave their approval for the merger. Europe, however, has opened an in-depth probe on concerns that the deal would cut competition and lead to higher pigment prices.