Shares in India\'s Kingfisher Airlines ended nearly five percent down on Friday, as two of its main lenders said the ailing airline had defaulted on interest payments on its outstanding loans. India\'s aviation regulator has raised safety concerns over debt-hit Kingfisher\'s operations, fearing that its financial problems could impact safety standards. Shares in the private airline tumbled as much as 10.76 percent to an intraday low of 18.25 rupees at the Bombay Stock Exchange, before ending at 19.45, down 4.89 percent. India\'s largest commercial bank and one of Kingfisher\'s biggest lenders, the State Bank of India, on Friday called its loan to Kingfisher \"substandard\" because of overdue payments. \"The airline has 90 days to pay and get the loan account reclassified as standard,\" SBI chief financial officer Diwakar Gupta told Dow Jones Newswires. Overdue bank loans in India are classified as \"bad\", \"substandard\" or \"doubtful\", depending on how long the loan remains unpaid. Another lender, the state-run Bank of India, also said it had classified loans to Kingfisher as \"substandard\" because of unpaid interest. Adding to the embattled group\'s woes, the Directorate General of Civil Aviation (DGCA) gave Kingfisher a deadline of Monday to come up with a timeline to address serious safety concerns raised in its audit report, it said Friday. The Times of India newspaper reported on Thursday that the DGCA audit suggested Kingfisher\'s licence may be under threat as its financial predicament could impinge on safety. Kingfisher strongly denied that safety was compromised and the regulator said there was no threat of operations being halted. The Bangalore-based carrier, which has seen its passenger market share slump in recent months, is battling a cash-flow crisis it says is caused by soaring fuel costs and high local sales taxes as well as a domestic price war. The airline shelved its low-cost Kingfisher Red brand in September in favour of more lucrative full-fare services, axed unprofitable domestic routes and has grounded 15 aircraft because it was was unable to pay maintenance fees.