By Mehreen Zahra-Malik and David Brunnstrom ISLAMABAD/WASHINGTON (Reuters) - Pakistani Prime Minister Nawaz Sharif will tell U.S. President Barack Obama this week that Islamabad will not accept limits on its use of small tactical nuclear weapons, Pakistani officials said on Wednesday. ****Pakistan insists smaller weapons would deter a sudden attack by its bigger neighbor India, which is also a nuclear power. Sharif and Obama are due to meet at the White House on Thursday.