After the Saxon attack on Riga in February 1700 Swedish officials and diplomats had wondered about the position of Russia. Would the Czar join Augustus or remain neutral? The reports from the Swedish representatives in Moscow, Pskov and Novgorod were inconclusive. They often reported such matters that could be considered as preparations for war, but on the other hand the Czar and his officials made every effort to appear friendly towards Sweden - including sending the envoy Chilkov who had his first meeting with Charles XII just as the war was declared in Moscow. It was also believed that a peace or an armistice with the Ottomans was far off, something that seemed to be confirmed by news from Constantinople which the Swedes received through contacts with the Sapiehas in Lithuania.
Possibly the first person to present the Swedes with conclusive evidence of the Czar's plans was the French envoy Charles-François Caradas, Marquis du Héron (1667-1703). On 1 September 1700 he came to see General Otto Vellingk in the camp at Rujen (Latv. Rujiena). Du Héron showed Vellingk a translation of the Czar's letter to King Augustus, dated Moscow 9 August (printed as no 325 in the first volume of Pisma i bumagi). This message reached Stockholm on 15 September. On 11 September Vellingk reported more disturbing news in a letter which reached Charles XII in Karlshamn on the 19th. Lieutenant Thilou at Neuhausen (Est. Vastseliina) and Captain Ringenheim at Sagnitz (Sangaste) had informed Vellingk that the border was so heavily guarded that no real news got through, but Russians had told them that the Swedish representative in Pskov had been arrested. However, Vellingk hoped that this only meant that the Czar was prepared to assist Augustus in forcing Sweden to agree to a peace. It was first on the 19th that Vellingk got the news from Narva that Russian forces had crossed the border, a letter which by way of Stockholm reached Charles when he already had arrived at Pernau.
Source: Riksarkivet, Skrivelser till Konungen. Karl XII., vol. 30. Letters from Otto Vellingk, September 1700-1705