"We have had engagement with the advisers from the company," said Neil McDonald, a partner at law firm Kirkland & Ellis, which had earlier released a letter on behalf of bondholders complaining about a lack of talks between the two sides.

"We continue to remain hopeful that they will engage constructively. But we will take appropriate steps to protect the rights of creditors (if necessary)," he added.

(Reporting by Marc Jones and Kirstin Ridley; Editing by Rachel Armstrong)