(Adds reason for half-yearly profit drop, details)

By Matt Smith

DUBAI, Aug 3 (Reuters) - Industries Qatar reported a 20 percent increase in second-quarter profit on Tuesday that beat analyst forecasts, although its half-year profit fell because of declining petrochemicals prices versus a year earlier.

The petrochemicals, steel and fertiliser producer made a profit of 1.5 billion riyals ($412 million) in the three months ending June 30, it said in statement.

This was up from 1.25 billion riyals a year ago, according to Reuters calculations based on earlier financial statements.

Analysts polled by Reuters had on average forecast Industries Qatar would make a quarterly net profit of 1.21 billion riyals.

The company's quarterly revenue rose 20 percent to 1.6 billion riyals versus the first quarter of 2015 as it sold more steel and petrochemicals.

Industries Qatar's half-year profit was 2.4 billion riyals, down from 2.8 billion riyals in the first half of 2014.

It attributed the half-yearly profit drop to declines in petrochemical product prices following a sustained slump in oil prices, which began in mid-2014.

Oil and petrochemicals prices are closely correlated and Gulf producers benefit from subsidised feedstock costs. This advantage over rival manufacturers not receiving subsidies lessens as oil prices decline while also compressing Gulf producers' margins.

Fertiliser prices also fell year-on-year in the first six months of 2015, Industries Qatar added.

($1 = 3.6403 Qatar riyals)

(Reporting by Matt Smith; Editing by Olzhas Auyezov and Louise Heavens) ((matt.smith1@thomsonreuters.com; 00971506354039; Reuters Messaging: matt.smith1.thomsonreuters.com@reuters.net))