UPDATE 1-Absa sees H1 headline EPS 15-25 pct lower

* Absa Bank H1 headline EPS seen down 25-35 pct

* Says interest margins contracted, portfolio value down

(Adds details)

JOHANNESBURG, June 23 (Reuters) - South Africa's Absa Group Ltd (ASAJ.J) expects first-half headline earnings to fall by up to 25 percent compared with the year-ago period, citing a challenging economic environment.

Absa said on Tuesday the group's headline earnings per share for the six months to end June would fall by between 15 to 25 percent.

"The financial performance of the group had been impacted by increasing impairment levels, a contraction of interest margins and a reduction in the value of investment portfolios," the bank said in a statement.

Headline earnings per share at Absa Bank, the country's largest retail bank, are expected to fall by between 25 to 35 percent for the period, while earnings per share at the unit are seen dropping by between 35 and 45 percent.

Absa is due to report interim results on Aug. 3. (Reporting by Agnieszka Flak)