Two of Chinas biggest cities have tightened mortgage rules in a bid to rein in escalating property prices and keep banking risks in check as new loans to home buyers soar.

Home prices in some big cities have risen rapidly this year as mortgage values have surged， stoking worries of growing bubbles in the real estate market and fears that credit support for other parts of the economy might be affected.

Authorities in Shanghai， the countrys commercial hub， announced on Monday （Nov. 28） they will raise down payments for firsttime buyers to 35% from 30%， according to a statement on the website of the citys housing and urban-rural development commission.

In addition， the policy， which went into effect on Tuesday， tightened the definition of first-time buyer to exclude those who had pre- viously taken out a mortgage， even if the property had been paid off and sold.

In the past， a person was deemed a first-time buyer as long as they didnt own a property in Shanghai，even if they had owned a property in the city but had sold it and cleared the mortgage.

The northern port city of Tianjin also announced rules on Monday to increase down-payment requirements for both first and second properties.

New-home prices in October rose by 31.1% in Shanghai and 25.3% in Tianjin from a year ago， figures from the National Bureau of Statistics showed.

Lending curbs issued by the two metropolises followed similar rules published earlier by cities that included Beijing and the southern boomtown of Shenzhen.

Surging mortgage values have alarmed policy makers this year. The Economic Daily， a newspaper run by the State Council， Chinas cabinet， said on Monday that the rapid rises in real estate prices this year were fueled by the huge amount of credit being offered to home buyers， which was "abnormal."

"Too rapid mortgage growth will increase credit risks at banks，" said a commentary written by an official with the banking regulators office in the northern province of Hebei.

"The huge loans made to the property sector by banks will also undermine banks credit support for the real economy." the article said.

Chinese lenders gave out 3.6 trillion yuan （$520 billion） in mortgages in the first nine months of the year， double the amount recorded in the same period last year and accounting for more than 35% of total new loans， central bank data showed.

Adding to concerns is the high loan-to-deposit ratio — a key indicator of the banking systems ability to cover fund requirements — at big banks in the country， which has benefited from decades of enormous household savings that have helped to keep lending interest rates low.

S&P Global Ratings in a report in October estimated that the adjusted loan-to-deposit ratio at Chinas top 50 banks， which includes corporate credits in off-balance-sheet wealth management products in loans， was about 80% at the end of June， no- ticeably higher than their reported ratios and exceeding the regulatory cap of 75%. The rate at many national and regional banks had already come close to 100%， it added.

"The next two to three years is a crucial window for China to rein in the ratio， or we will be in serious trouble，" an earlier Bloomberg News report quoted S&Ps Beijing-based director Liao Qiang as saying.

"Reaching 100% doesnt mean a crisis will ensue immediately， but it shows Chinas entire deposit base is used up and any loss of confidence from savers will severely destabilize the banking system."