Ilva has been under state-supervised special administration since 2015, after magistrates said it must be cleaned up or closed. ArcelorMittal agreed last year to buy it after outbidding competitors and promised to fix the problems.

It was due to take over the business on July 1, but the newly installed Italian government delayed the handover and has pressed ArcelorMittal to come up with a much better plan for the firm.

In a statement issued on Friday, ArcelorMittal said it would unveil its new proposals in the coming days.

"We have worked to increase the environmental and jobs plan, which will improve the original commitments and strengthen the environmental performance of Ilva and its support to local communities," it said in a statement.

"Once an agreement has been reached on the basis of this project, we are confident that all the parties, including the unions, will be satisfied and will be ready to conclude the deal quickly", it added.

The group said it had already promised to invest 4.2 billion euros (3.75 billion pounds)in Ilva, including 1.2 billion to boost productivity and 1.15 billion to curb pollution. It has also told unions it plans to reduce the workforce in Ilva's main Taranto plant to around 7,600 from 10,900 now -- a proposal the unions have rejected.

ArcelorMittal added that it had taken part in the government tender in a "clear and transparent manner".

It released its statement shortly after Deputy Prime Minister Luigi Di Maio, who also serves as industry and labour minister, questioned how the tender was awarded, calling the process a "mess".

Earlier this month, Di Maio asked Italy's anti-corruption unit to review the bidding process. "It has confirmed that our concerns about the Ilva tender procedure were well founded," the deputy prime minister told parliament on Friday.

He provided no details and gave no indication that he was considering pulling the contract from ArcelorMittal, but said he would seek an internal ministry investigation into the deal.

Di Maio's anti-establishment 5-Star Movement had promised ahead of a March national election that it would shut down Ilva if it took power because of environmental concerns.

Former industry minister Carlo Calenda, who oversaw the original tender, rejected Di Maio's criticism.

"(Di Maio) has said serious and false things in parliament," Calenda wrote on Twitter. "If you think the bid was rigged, assume your responsibility and cancel it."

(Writing by Crispian Balmer; Editing by Kirsten Donovan)

By Angelo Amante