(Adds comparison with expectations; further details)
* Output growth rate below some analyst estimates
* Misses own forecast for annual output growth for 2011-2015
* Capex spending set at $17.4 billion-$19.8 billion
HONG KONG, Jan 20 (Reuters) - Top Chinese offshore oil and gas producer CNOOC Ltd said on Monday it is aiming for an up to 4.3 percent output increase this year, excluding contributions from Canadian oil producer Nexen that it acquired a year ago.
The targeted growth rate is below the expectations of some research houses, including Barclays, which had been looking for a 6 percent increase for this year. CNOOC, once an investor darling for its high-growth profile, has been struggling to boost its output over the past few years as domestic fields age.
CNOOC's shares were one of the worst performers among major exploration and production companies in 2013 due to worries about its output and the premium it paid to acquire Nexen.
CNOOC's 2014 production estimate would for the second year in a row put the state producer behind the 6-10 percent compound annual output growth it has forecast for 2011-2015. The five-year target excludes output from Nexen as well.
CNOOC has said the acquisition of Nexen boosted its production by 20 percent and proven reserves by 30 percent.
Excluding contribution from Nexen, the state-run company plans to produce 353 million-366 million barrels of oil equivalent (boe) this year, compared with estimated output of 351 million boe in 2013, it said in a Monday filing with the Hong Kong stock exchange.
The 2013 output estimates were above the company's target of 338 million-348 million boe.
CNOOC also said it has earmarked 105 billion yuan to 120 billion yuan ($17.4 billion to $19.8 billion) in total capital spending for exploration, development and production for this year, with Nexen accounting for about 19 percent.
CNOOC acquired Nexen for $15.1 billion in March 2013 in China's biggest foreign corporate takeover.
Including Nexen's output, CNOOC aims to produce 422 million-435 million boe this year, up from 412 million boe estimated for 2013.
The 2014 output forecast was based on an assumption that international crude benchmark Brent would average $102.0 per barrel this year, compared with an average oil price of $108.8 assumed for 2013, CNOOC said.
Shares of CNOOC rose 0.72 percent on Monday ahead of the announcement. The stock lost about 13 percent in the last 12 months.
(Reporting by Charlie Zhu; Editing by Tom Hogue)