Russian authorities are holding back from permitting UTair to operate Airbus A321s, claiming the airline needs to improve its safety management capabilities first.
UTair has 20 A321s on order, a deal signed in July 2012, and had expected deliveries to begin this year.
The company already has a diverse fleet - covering some 600 aircraft, of which about a third are airliners - spread across different activities. It operates some 350 helicopters.
Last year it introduced its first Boeing 767, adding to an airliner base which also includes Boeing 757s and 737s, Bombardier CRJs, various ATRs and a range of Soviet-era types.
UTair told investors at the end of May that its rapidly-expanding business would include another 46 aircraft and 96 helicopters by 2016.
"Active fleet development will allow us to become a leading Russian airline with the youngest and most modern fleet in Russia by 2020," says chief executive Andrey Martirosov.
Alongside agreements to acquire three 767-200ERs the carrier completed deals for four 737-800s, six 737-500s and six ATR 72s.
But the fatal loss of an ATR 72 in April 2012 and a number of helicopter accidents have led federal air transport authority Rosaviatsia to express concerns about the company's ability to introduce another new aircraft type.
"The safety management system of UTair cannot cope with a large heterogeneous fleet, dispersed not only across many airports of Russia but also in other countries," it states.
Without further effective measures to ensure flight safety, it says, the introduction of new aircraft types to the air operator's certificate could become an "unbearable burden" for the safety management system.
It says it will not consider any application to include the A321s - to be powered by CFM International CFM56s - until the airline has taken measures to improve the situation.
UTair, which is based at Khanty-Mansiysk, says it is undertaking preparatory measures for staff training as well as maintenance in order to ensure that it complies fully with Russian and international standards.
It adds that it is providing all necessary materials to the regulatory authorities, and putting in place recommendations during May-June following a Rosaviatsia safety audit in April. The airline says it is "confident" that its plans will be "successfully implemented".
Thirty-seven aircraft - including Tupolev Tu-154s and a batch of ATRs - will be withdrawn from the fleet this year. UTair is optimistic that it will be able to begin A321 operations in July, followed by services using Sukhoi Superjet 100s, another new type, in the fourth quarter of 2014.