Evergrande is the world's most indebted property company, with more than $300 billion in total liabilities, which include nearly $20 billion of international bonds all deemed to be in default after a run of missed payments late last year.
The creditor group, represented by law firm Kirkland & Ellis and investment bank Moelis, said in a statement it had to "seriously consider" enforcement action after a lack of engagement by the firm at the heart of China's property crisis.
One of the creditor group's advisers later told Reuters that Evergrande's team had been in communication in response to the statement, which had stressed it was prepared to take "all necessary actions" to defend its legal rights.
Evergrande and its advisers at U.S. investment bank Houlihan Lokey declined to comment.
The property giant last month set up a risk management committee, including officials from Chinese state entities, and it has repeatedly said that this group and the company itself would actively engage with creditors and protect the legitimate interests of the parties involved.
"Actions speak considerably louder than words," the creditor group said in its statement, adding that the overriding impression was that Evergrande "has disregarded its offshore creditors and the legal rights of its creditors".
Despite efforts to engage in substantive dialogue, the group said, it has "received little more than vague assurances of intent, lacking in both detail and substance" and urged Evergrande not to make any asset sales without consulting first.
"The AHG (ad-hoc group) expects and calls on the directors (of Evergrande).. to strictly comply with their fiduciary duties which require them, amongst other things, to have regard to and act in the best interests of their creditors".
(Graphic: Evergrande's woes hammered Chinese high yield debt markets,
Evergrande's international market bonds are lower in the legal pecking order than Chinese market bonds that the company has been fighting hard in recent weeks to keep out of default.
The international debt is what is known as 'unsecured' and issued by a Hong Kong offshoot, meaning creditors do not automatically have the right to seize anything on the mainland, where Evergrande has almost all of its 1,300 projects.
Those bonds are trading at just 10-15% of their original value, meaning that their owners may feel they now have little left to lose by taking legal action.
The first step down that path would be to 'accelerate' a bond still with a future repayment date. That would then see the bond's 'trustee' order Evergrande to pay the debt immediately.
If the developer failed to come up with the money, a 'winding up' order could be made against Evergrande's Hong Kong entity that issued the debt. It would open a host of new options, although it would still be difficult for the creditors to claim any assets in mainland China.
"We continue to remain hopeful that they will engage constructively," Neil McDonald, a partner at Kirkland & Ellis told Reuters, adding: "But we will take appropriate steps to protect the rights of creditors (if necessary)".
The price of Evergrande's defaulted bonds barely budged after the statement. Its shares had gained 4.65% in Hong Kong as Chinese property stocks extended a rebound triggered by signs that Beijing is looking to ease some pressure on developers.
(Additional reporting by Kirstin Ridley in London; Editing by Muralikumar Anantharaman, Jason Neely and Alexander Smith)
By Clare Jim and Marc Jones