* UK thwarted 7 plots in last year, says Cameron
* PM says "generational struggle" needs more manpower
* More cash allocated for aviation security (Adds Cameron speech comments)
By Kylie MacLellan
BELEK, Turkey, Nov 16 (Reuters) - Britain will increase its intelligence agency staff by 15 percent and more than double spending on aviation security to defend against Islamist militants plotting attacks from Syria, Prime Minister David Cameron said on Monday.
Britain said it had decided to bolster its defences following a growing number of plots against it and the attacks in Paris, Tunisia and elsewhere.
Cameron said British security services had foiled seven potential attacks in Britain over the last year and more manpower was needed to combat a "generational struggle".
"We need to do more to ensure our agencies have the resources and the information they need to prevent and disrupt plots against this country at every stage so ... we will make a major additional investment," Cameron said, according to a text of his speech released by the prime minister's office.
Speaking in London after attending a meeting of G20 leaders in Turkey where security issues dominated, Cameron said Britain would demonstrate the same resolve in the fight against terrorism as it showed against Nazi Germany in World War Two.
"It is that same resolve that will defeat this terrorism and ensure that the values we believe in, and the values we defend, will again in the end prevail," he said.
As part of its broader five-year defence and security review, which is due to be published on Nov. 23, Britain will fund an extra 1,900 officers at its MI5 and MI6 spy agencies and the GCHQ eavesdropping agency, Cameron said.
It will also spend 2 billion pounds ($3 billion) by 2020 on boosting the capabilities of British special forces, including investing in communications equipment, weapons and vehicles.
The British leader defended his recent meetings with Egyptian President Abdel Fattah al-Sisi and Chinese President Xi Jinping, which were criticised by some in Britain due to those countries' human rights records.
"You can't conduct foreign policy by press releases and pious statements in parliament. You have to engage ... A deeper partnership means a deeper conversation and a greater ability to address the issues that concern us," he said.
Earlier on Monday Cameron pledged to boost aviation security following the crash of a Russian airliner in Egypt last month, which Britain has said it believes was brought down by a bomb.
He also ordered a rapid review of security at several airports around the world, in particular in the Middle East and North Africa and airports through which high numbers of British citizens travel.
The assessments, due to be conducted over the next two months, will focus on measures such as passenger screening, physical security at the airport, hold baggage and freight screening.
Additional security measures put in place at potentially vulnerable airports over the past year will also be reviewed and on Tuesday the National Security Council will discuss British aviation security policy.
Cameron said he planned to more than double government spending on aviation security, currently around 9 million pounds a year, over the next five years.
This new funding will provide extra aviation security experts to regularly assess security at airports around the world as well as advice, training and equipment for other countries to help them increase security at airports.
It will also fund research into screening technology and to detect new threats, the government said. ($1 = 0.6567 pounds) (Editing by Guy Faulconbridge and Gareth Jones)