UPDATE 2-ECB's covered bond programme makes slow start

* Eurosystem has spent 23 million euros on bond purchases

* Small amount not definite signal of problems with approach

* Overnight deposits down, but still elevated (Adds details, analysts)

By Sakari Suoninen and Jane Merriman

FRANKFURT/LONDON, July 10 (Reuters) - Euro zone central banks made a sluggish start to their major bond purchase programme, buying just 23 million euros' ($32 million) worth of covered bonds, ECB data showed on Friday.

But analysts said the slow start to the programme, one of the European Central Bank's main unorthodox measures to revive the euro zone economy, did not necessarily mean there was an underlying problem in its approach.

The ECB and national central banks of the 16-country currency area need to pick up the pace to reach the targeted 60 billion euros in the next 12 months. They would have to spend about 240 million euros each working day to reach it.

Natixis economist Sylvain Broyer said he expected the programme to steam ahead. "The ECB will give continuous shocks, not huge shocks, but will simply breathe life to that market," he said. "It will be on this market continuously for a year."

The central banks will buy euro-denominated covered bonds -- debt backed by a pool of assets such as mortgage loans that remain on the issuing bank's balance sheet -- directly from primary and secondary markets over the next year.

Despite the leisurely pace, the programme has already affected covered bond markets, analysts said.

"The impact of the ECB programme was quite significant before they even started buying," Heiko Langer, a BNP Paribas analyst said. "But spreads accelerated their tightening this week in the secondary market. Also, bonds left on the sidelines have moved significantly."

Tighter spreads and the ECB's desire to avoid market distortions mean the bank was likely to buy more in the primary market than in the secondary, Langer also said.

The modest amount of bonds purchased made drawing conclusions difficult this early. "The number is so small it's hard to interpret," said Franz Rudolf, a director in credit research at UniCredit. "It is the size of a single trade and a small one."

In its first disclosure of bonds bought under the programme, the ECB did not offer details of the purchases but only gave the total amount spent by July 9. The amounts are published on Reuters information page ECB59 after the purchases settle, which takes 2-3 days.

DEPOSITS ELEVATED