Mexico's state-owned oil firm Pemex has posted a 53bn-peso (US$4.09bn) second-quarter loss, almost 7% wider than the year-ago period.

In a statement, the company said earnings had been hit by increased operating costs and inventory acquisitions.

Despite the deficit, Pemex reported a 4.1% increase in Q2 revenue to 409bn pesos.

During an investor conference call, Pemex's chief financial officer Mario Beauregard cited tax liabilities as an obstacle to profitability.

The company paid 218bn pesos to Mexico's treasury during Q2, about 5% more than the same period last year.

Crude production from April to June was 2.45Mb/d, down from its 2013 average of 2.52Mb/d.

The Q2 results came as Mexico's congress deliberates over secondary legislation that will open the country's energy sector to private investors.

The reform is expected to provide international firms with access to deepwater and unconventional plays as soon as next year.

"Pemex needs to redefine its future strategy as a company that generates value and not as a state monopoly," Pemex added in the statement.