The declaration at the end of the two-day Group of Eight summit sought to narrow the diplomatic chasm between Assad's key backer, Russia, and Western leaders on starting peace talks in Geneva to end a two-year civil war that has claimed an estimated 93,000 lives.
G-8 leaders also published sweeping goals for tightening the tax rules on globe-trotting corporations that long have exploited loopholes to shift profits into foreign shelters that charge little tax or none. But that initiative, aimed at forcing the Googles and Apples of the world to pay higher taxes, contained only aspirations, no binding commitments.
The Syrian declaration said the country needs a new coalition government with "a top leadership that inspires public confidence." It made no reference the possibility of sending U.S., British or French weapons to rebels, an option being kept open by all three G-8 members. Russia refused to back any declaration that made Assad's ouster an explicit goal, arguing that it would be impossible to start peace talks with a predetermined outcome.
Reflecting the profound divisions that remain, the British host, Prime Minister David Cameron, declared in response to reporters' questions it was "unthinkable that President Assad can play any part in the future government of his country. He has blood on his hands. He has used chemical weapons."
Putin, speaking simultaneously to Cameron at a different location in a gesture that some diplomats construed as rude, rejected Cameron's views as unproven.
And referring to last month's butchery of an off-duty British soldier in London by ax- and knife-wielding Muslim extremists, Putin warned Cameron that the weapons sent to Syria might end up being used to kill people in Europe.
"There are many such criminals in the ranks of the (Syrian) opposition, such as those who committed the brutal murder in London. Do the Europeans want to provide such people with weapons? ... We are calling on all our partners to thoroughly think it over again before taking this very dangerous step," Putin said.
Reflecting growing unease at the behavior of Muslim extremists in the ranks of Syria's splintered opposition forces, the G-8 declaration said participants in any peace talks must agree to expel al-Qaida-linked fighters from the country.
The declaration condemned human rights abuses committed by government forces and rebels alike, and called on both sides to permit access by U.N.-led chemical weapons experts trying to investigate the contentious claims of chemical weapons use.
In its only concrete commitment, the plan commits a further $1.5 billion in aid for Syrians driven from their homes by the conflict: 4.2 million within Syria and 1.6 million more taking refuge in neighboring countries. The G-8 noted that the new funds would cover only part of the United Nations' 2013 appeal for $5.2 billion in Syria-directed aid.
Earlier, G-8 leaders announced new goals to combat tax avoidance by multinational companies. In a joint statement they said tax authorities should share information "to fight the scourge of tax evasion" and make it harder for companies to "shift their profits across borders to avoid taxes."
Britain heralded the agreement as a good first step toward creating a new environment of corporate transparency. A key principle in the plan would require multinationals to declare how much tax they pay in each country.
U.S. Senate hearings this year investigating the tax payment policies of Apple found that the smartphone and computer innovator also has developed some of the world's most innovative tax-avoidance policies. Apple admitted it used, legally, two companies registered in Ireland - but in one case managed from the U.S. state of Nevada - to manage much of the company's non-U.S. profits worldwide and paid taxes at a rate of less than 1 percent.
British lawmakers likewise have sharply criticized Google UK for registering all of its regional sales in neighboring Ireland, which charges half the rate of corporate tax.
Many of the world's leading companies, and even bands like U2, employ complex corporate structures involving multiple subsidiaries in several countries to minimize the tax bills in their home nation. One such maneuver, called the "double Irish with a Dutch sandwich," allows foreign companies to send profits through one Irish company, then to a Dutch company and finally to a second nominally Irish company that is headquartered in a usually British tax haven.
Campaigners for greater tax transparency appealed to the G-8 to ensure that reforms benefited the poorest countries of Africa, South America and Asia as well as the rich West. Anti-poverty campaigners stressed that shell companies provide a key mechanism for embezzling government funds in corrupt countries.
Cameron says Britain will lead by example by creating a registry of who really owns companies, and will consider making it public - an idea viewed skeptically by many other countries fearful of scaring companies out of their jurisdictions.
And Britain itself stands accused of being one of the world's main links in the tax-avoidance chain. Several of Britain's own island territories - including Jersey, Guernsey and the British Virgin Islands - serve as shelters and funnel billions each week through the City of London.
"Of course, Britain's got to put its own house in order," said Britain's treasury chief, Chancellor of the Exchequer George Osborne, who addressed the G-8 meeting on corporate tax reform along with International Monetary Fund Managing Director Christine Lagarde.
She commended the G-8 initiative as necessary to address taxpayers' indignation at corporate tax-dodging and reflected the fact that "almost all governments need additional revenues."
Before the summit, Britain announced a provisional agreement with the finance chiefs of nine British offshore dependencies and territories to improve their sharing of information on individuals and companies banking cash there.
Also Tuesday, Cameron won commitment from all eight G-8 nations to stop paying ransoms to kidnappers in hope of deterring the practice following January's bloody capture by al-Qaeda-linked militants of an Algerian gas facility. Ten Japanese, five Britons, three Americans and a French national were among the 40 civilians killed as Algerian forces retook the facility.
Hostage-taking of foreign workers for cash payments is on the rise across much of West Africa, particularly Nigeria with its own oil industry dominated by Western companies and foreign managers. Italian and French authorities have been accused of paying ransoms on the quiet.
Cameron invited the leaders of Libya and the African Union to join the talks over lunch to discuss the issue.
Associated Press writers Vladimir Isachenkov and David McHugh contributed to this report.