* ECB cuts rates below zero to buoy euro zone economy
* Sprint agrees to buy T-Mobile US - source
* Futures up: Dow up 26 pts, S&P 2 pts, Nasdaq 3 pts (Updates prices after ECB decision)
NEW YORK, June 5 (Reuters) - U.S. stock index futures were slightly higher on Thursday after the European Central Bank cut interest rates to record lows while indicating further monetary policy actions will follow.
* Seeking to combat deflation, the ECB cut the deposit rate to -0.10 percent, the main refinancing rate to 0.15 percent, and the marginal lending rate - or emergency borrowing rate - to 0.40 percent. Further monetary policy measures will be announced later in the day, the ECB said.
* The widespread expectation of the monetary policy move had kept pressure on the euro in recent days, while keeping yields on government bonds low. Earlier on Thursday, Spain auctioned 3- and 5-year bonds at record low yields.
* S&P 500 e-mini futures edged up 2 points and fair value - a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract - indicated a slightly higher open. Dow Jones industrial average e-mini futures rose 26 points and Nasdaq 100 e-mini futures added 3 points.
* Sprint has agreed to pay about $40 per share to buy T-Mobile US, a person familiar with the matter told Reuters on Wednesday, marking further progress in the attempt to merge the third and fourth-biggest U.S. mobile network operators. Sprint shares were up 3.2 percent in premarket trading while T-Mobile US rose 0.4 percent.
* Ciena Corp shares jumped 14 percent premarket after the company posted earnings that beat expectations and gave a revenue outlook above forecasts.
* On the data front, the Labor Department's weekly reading on applications for unemployment insurance is due at 8:30 a.m. EDT (1230 GMT). The number is expected to rise to 310,000 from 300,000. (Reporting by Rodrigo Campos; Editing by Nick Zieminski; Editing by Chizu Nomiyama)