(Reuters) — AerCap,  the world's biggest aircraft lessor, said Monday it was inconceivable that it would not recover some losses from insurers over jets stranded in Russia, as lessors took their battle for redress to London’s High Court.

More than 400 aircraft, worth almost $10 billion, have been stuck in Russia after Western countries slapped sanctions on the country over the war in Ukraine and Russian aircraft lessees did not return the jets.

Insurers are balking at payouts, alleging in part that there has not been a physical loss of the planes yet, that the jets and engines are no longer subject to a lease agreement and that Western sanctions prevent them from providing cover.

Dublin-based AerCap said it was out of pocket to a "colossal" degree and, along with at least four peers, has lodged a lawsuit that hinges on whether the alleged loss of the aircraft has triggered war-risk insurance policies, which have payout limits, or uncapped all-risk policies.

AerCap, the largest claimant, is suing insurers such as American International Group Inc. and Lloyd's Insurance Co. for $3.5 billion over the loss of 116 aircraft and 23 engines under its all-risks insurance policy. Alternatively, it is claiming $1.2 billion under its war-risks policy, court filings show.

AerCap, Dubai Aerospace Enterprise, Merx Aviation, KDAC Aviation Finance and Falcon allege they terminated Russian leasing contracts in line with sanctions and demanded in vain for the return of aircraft and engines, court documents show.

DAE and Falcon are suing 11 insurers, including Lloyd's of London, AIG, Chubb Ltd. and Swiss Re over 21 aircraft and equipment valued at $900 million under all-risks policies, or for just under $800 million under war-risks policy limits.

Merx is claiming more than $255 million over the alleged loss of six aircraft, and their engines and records, while KDAC is suing for $21.5 million over the loss of one jet, court filings show.

Judge Christopher Butcher said Monday that the five lessors' cases should be heard together — though the inclusion of Merx and KDAC will remain under review — in what AerCap's lawyers described as a “mega trial” that is expected to begin in October 2024.