Quarrels have broken out behind the scenes of Anthem Inc.'s (ANTM.N) proposed acquisition of Cigna Corp (CI.N), as the health insurers seek regulatory approval for their landmark deal, the Wall Street Journal reported Sunday.

The newspaper said the squabbles could delay or derail antitrust approvals, though the deal does not appear to be in danger of imminent collapse. Sources familiar with the matter confirmed the citations in the Journal story. 

An Anthem spokeswoman declined to comment on the Journal report. Cigna officials were not immediately available to comment. 

In a series of letters between top officials including their chief executives, the Journal said Anthem and Cigna accuse each other of violating the July merger agreement and fumbling submissions to regulators.

A key point of contention is Anthem's lawsuit against Express Scripts Holding Co (ESRX.O), a middleman for prescription drugs, according to the newspaper. The suit accuses Express Scripts of overcharging Anthem for drugs.

Cigna Chairman Isaiah Harris Jr., in an April 9 letter to Anthem's board, said the suit could hurt the prospects for regulatory approval and the combined company's value, the Journal said. Anthem responded to Cigna's board that it had alerted Cigna to the possibility of a lawsuit early in their merger talks, and that getting better prices from Express Scripts could only be beneficial, the newspaper reported.

Disclosure statements from the two companies earlier this month hinted that they had different views about when the deal would close. 

In a filing with the Securities and Exchange Commission Cigna said that the deal may close in 2017, rather than in 2016 due to the complexity of the regulatory process.

Anthem, meanwhile, said it had not changed its expectations on timing.