The top share index rebounded early on Tuesday having fallen nearly six percent over the last six days, with investors buying on the dips, as traders said concerns over debt problems in Europe and United States would make any upside short-lived.

Financials <.FTNMX8350> <.SXIP> and miners <.FTNMX1770>, the sectors that have led falls in the past week, rallied hardest, up as much as 1.7 percent, helping the FTSE 100 <.FTSE> gain 40.49 points, or 0.8 percent to 5,263.09 by 0901 GMT.

Traders, however, said gains looked fragile as there was little support from fund managers or institutional investors with so much gloom surrounding the global economy.

Most contracts being picked up by clients today in mining and banking stocks are very short term, showing that a high degree of investor sensitivity remains, a London-based trader said.

Terry Pratt, institutional dealer at IG Index, said: There's still this double whammy of concerns squaring up to the investment community right now, citing the U.S.'s failure to compromise over deficit reduction plans and Europe's debt crisis weighing on global recovery prospects.

Reflecting wider sentiment over Europe's debt problems, Italian and Spanish bond yields remained uncomfortably high.

Against this backdrop it's difficult to see where the support is going to come from, IG's Pratt said.

There was marginally better news in the United States with ratings agencies Standard & Poor's and Moody's saying the deficit committee's failure would not trigger an immediate downgrade of the U.S. credit rating.

And rebounding metals helped mining stocks higher as investors hoped that continuing demand from China will help support the sector.

China's daily apparent demand for refined copper inched up 0.1 percent month on month in October and surged 42.6 percent from a year earlier, data showed.

Europe's second-biggest tour operator Thomas Cook slumped more than 70 percent as it said it would delay publication of its full-year results, originally due on Thursday.

Thomas Cook, which issued a string of profit warnings leading to the departure of its chief executive in August, also said it could breach December banking covenants following a further deterioration in its trading and cash position as it battles the bleak economic backdrop.

BAE Systems shed 1.2 percent as JP Morgan cut its target price on the defence contractor saying it continues to see reasons not to buy the firm's shares. BAE is unlikely to outperform its peers until it can answer the question, what happens to growth post 2013.

Smith & Nephew dropped 2.5 percent, as Citigroup repeated its sell rating on Europe's largest artificial hip and knee maker on concerns over its growth outlook.

We leave FY11 sales unchanged but lower future years by 1 percent ... We feel the orthopaedic industry is going ex-growth with double-digit earnings growth no longer possible.

In terms of domestic economic data, UK October public sector finance figures are due at 0930 GMT.

(Editing by Erica Billingham)