On the final stop in his foreign tour, U.S. presidential candidate Barack Obama said he had a "terrific conversation" with British Prime Minister Gordon Brown in London.
He said a strong trans-Atlantic relationship was needed to deal with a wide range of world issues, including Iraq and Afghanistan.
The Illinois senator also defended his decision to travel to Europe and the Middle East, saying problems encountered by Americans at home are often best dealt with by working with allies overseas.
"I am convinced that many of the issues that we face at home are not going to be solved as effectively unless we have strong partners abroad," Obama said. "And unless we get a handle on Iraq and Afghanistan, not only are we going to be less safe, but it's also going to be a huge drain of resources."
Obama has called for the removal of U.S. combat troops from Iraq within 16 months of his taking office, should he win November's presidential election.
Obama reiterated that it was time for the United States and its allies to focus on Afghanistan.
"I have already committed to an increase in American troops in Afghanistan," he said. "Obviously we would like some of that burden shared. I think it's going to be necessary on order for us to complete the job that need to be done.
"Just as important is going to be rebuilding the infrastructure of that country, in some cases building infrastructure for the first time, creating greater capacity in the government, training police. That's all going to require a significant investment of resources."
He also called for a more effective use of resources in Afghanistan, "both on the military side and on the economic and social development side, so that having learned some lessons over the last five-six years about what's working and what's not, that we can move very aggressively to try to finish the job there."
Obama said his discussion with Brown also touched upon climate change, terrorism, and the financial markets.
Before the meeting, Obama had talks with Tony Blair. The former British prime minister represents the Middle East Quartet -- the European Union, Russia, the United Nations, and the United States -- which is driving negotiations for a lasting peace deal between Israel and the Palestinians.
The two also discussed Blair's work on climate change, an issue on which he has worked since leaving office in June 2007.
Obama is due to meet opposition Conservative leader David Cameron later, before returning to the United States.
He has visited Germany, where he gave a keynote foreign-policy speech in Berlin, and France, where he met President Nicolas Sarkozy.
While in Paris, Obama said Iran should not wait for the next U.S. president to be elected before resolving its dispute with the West. He said Tehran should promptly accept an international call to freeze its "illicit nuclear program." Iran insists its nuclear campaign is peaceful.
Obama's tour also took him to the Middle East where he visited Jordan, Israel and the Palestinian territories, as well as Iraq and Afghanistan.
with agency reports