By Thomas Grove

ISTANBUL, Dec 5 (Reuters) - The presidents of Pakistan and Afghanistan pledged on Friday to boost cooperation in the fight against Islamic militants along their shared border.

"Both countries are affected seriously by terrorism," Afghan leader Hamid Karzai told a news conference at the end of a day of Turkish-sponsored talks in Istanbul.

Afghanistan has long accused Pakistan of not doing enough in the fight against Taliban militants based in the tribal region, but ties have improved since Pakistani President Asif Ali Zardari took office.

Karzai said contacts between Pakistan and Afghanistan under Zardari had been "extremely good" and the two sides had agreed a joint strategy to fight al Qaeda and other militant groups.

Turkish President Abdullah Gul, who hosted the talks, said the attacks last week in India's financial capital Mumbai had added urgency to improving security cooperation in the region.

Zardari said Pakistan was investigating reports that Pakistan-based Islamic militants were responsible for the attack and said Islamabad was committed to fighting insurgents.

"We are aware of those reports and we are investigating them (...) We will continue our fight against perpetrators of terror and stateless actors of terror," Zardari said.

India has blamed the Pakistan-based Lashkar-e-Taiba group for the three-day rampage that killed 171 people. The Pakistani government and security officials have denied any involvement by state agencies.

The attacks have fueled concern that rising tension with India will divert Pakistan's attention from fighting insurgents in its western tribal areas who are infiltrating Afghanistan to stage attacks on U.S., NATO and Afghan forces.

"Pakistan is the victim of terrorism and Pakistan will continue its fight against terrorism in the same way after the attacks," Zardari said when asked about such concerns.

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